The best time to review a budding restaurant is probably not in the first week of its operation. Nine times out of ten, the initial fortnight can be bedevilled by a variety of hiccups from staff training lessons still to sink in, to noisy butt-cheek exposed joiners still completing their snagging list - not to mention various other bedding in and teething issues.

On the other hand, rarely do these ventures charge customers half price during those opening innings, so presumably they’re fair game for reviewers to visit. From the other side of the fence, certainly, the pressures of modern publishing put kudos - rightly or wrongly - on getting there and into print first, even when everybody knows that it isn’t the best period to judge an embryonic outfit.

But other incidentals can affect a review. In the run-up to Christmas, amid the commercial and consumerist deluge, restaurants often operate in a different manner. They can be beset with large groups, which would not normally descend. The kitchens can also opt for those horribly predictable special festive season menus, with the obligatory turkey with ‘all the fixings’, that also invariably jack up the price a quid or two.

Then there are the vagaries of the season. In Glasgow, the months of November and December last year were like living in a constant typhoon, with wind whipping up disjointed brollies like popcorn in a blow-dryer, while the rain fell rather constantly - or rather whistled by horizontally. Not necessarily ideal conditions to judge whether a place has caught on, either.

Finally, any food writers worth their seasoning should always visit incognito. Alas, after some years in the game, invariably one will get caught out occasionally by managers who have been it just as long. Not the end of the world, however: even play directors and band managers know when the critics are going to be in the house. The show must go on.

And with that, so it did go on amid the monsoons of mid-December at the recently opened branch of Ad Lib in the Merchant City. Residents will be familiar with the location: the old firehouse that once housed the Fire Station restaurant (anyone remember its haggis pasta dish?) and Gumbo after that.

Design alterations are not as dramatic as the changing footfall on eastern Ingram Street these days with the newly opened student flats on the High Street nearby. The venture feels as if it’s about five times the size of the flagship Ad Lib on Hope Street. The ceiling has been painted a softer, creamy hue, so that the place feels lighter and less cavernous. The raised seating area nearest the pavement has been taken down to ground level on a new parquet floor, with manager Angus Stewart plumping for more glazing to be added to those big street-side timber doors. A small bar (sans bar stools) sits on a platform to the left, while booths up three steps line the wall to the right. The kitchen’s a bit more visible, through hatches behind a service counter at the back. Otherwise the listed marble interiors remain, as does the big clock face (stuck at five past one), much as before.

Ad Lib’s nod to the American diner and classic Yankee recipes has always felt fairly assured, while not delving into the stereotypical depths of the mass-marketed chains. Chef Chris Orr has returned from some world travels and his menus show a continuing affection for the confederate states, too. So it’s hush puppies (cornbread dumplings), as well as a ‘soul food’ section with pan-fried red snapper, barbecue pork, and even a savoury pumpkin pie. US staples such as succotash (sweet corn and lima beans) salad or steak are complemented by blackened chicken skewers, smoked haddock and clam chowder or Texan chilli nachos.

Naturally, at the core of the menu, there are burgers. Actually, the billing is ‘gourmet burgers’ and no advertising standards appear to be violated with such a tag. Requested pink, the hand made patty is plump and juicy, nicely charred on the outside. The downfall of many a gourmet burger can be the bun, so substantial that only a person with an articulated jaw could possible obtain decent purchase. Here the floury bap is wide rather than obscenely proud. Even without any added toppings, the rich, well-seasoned Angus beef version is topped with an onion relish, slice of pickled gherkin, tomato and plenty of fresh salad leaves. Skinny chips (yawn) may not set the potato world alight, but they are rather infectiously inhaled.

‘Dirty rice’ is a spicy Cajun/Creole influenced side dish, often made with chicken giblets. Here the moist grains have been turned with some chicken stock and red beans, herbs and even some pork strands. The mouth-watering result is as satisfying as New Orleans famous son Ernie K Doe’s tune ‘Mother in Law.’ Carlsberg is the featured lager, but Birra Poretti on draught provides a more refined option.

With nascent enterprises, the financial pressure to open is often such that premises throw open the doors to generate some much needed revenue - at times before they’re truly ready. In Ad Lib’s inaugural week, one could find small things still in need of addressing (like the ‘F’ and ‘M’ scrawled on paper stuck to the loo doors). But by the time you see this, we’d expect any of these minor wrinkles to have been ironed out.

Ad Lib, 33 Ingram Street, Glasgow, 0141 552 5736.

Sample menu item: Morroccan lamb burger with tomato and coriander salsa: £8.75