Feng Sushi, 280 West End Lane, London NW6

You know that feeling when you've ordered so much food you have to put the candle and the flowers on the floor? That.

But my hot-cheeked shame at being so greedy is tempered by the realisation that our table at Feng Sushi is far, far too small. Two of us have ordered (deep breath) chilli edamame, sardines unagi-style donburi, deluxe sashimi, chilled ramen noodles, deep-fried tuna maki and spicy crispy baby squid.

At Feng Sushi, unlike some swisher Japanese restaurants, the portions are healthy, not dolls' house-sized. The deluxe sashimi, for instance, has salmon, tuna, scallop, yellow tail with chilli, and rainbow trout with salmon eggs on the plate, and not just one or two slivers of each, either. At £16, it's a winner. The sardines donburi, £7, has two plump Cornish fish on top of plenty of seasoned rice, and a heap of fresh-grated ginger on top of that. Deep-fried tuna maki – a devillish bastardisation of sushi and tempura – costs £9 and has 10 pieces.

But is Feng Sushi any good? The mini-chain (there are now eight branches across London) was founded by Silla Bjerrum in 1999; two years ago, the serial investor Luke Johnson bought in. In both scale and range, it's on the up. However, expansion like this sometimes means that quality heads down.

There's no point being snooty about a Japanese restaurant serving spring rolls and Asian fish'n'chips. If you want one artfully placed piece of hand-smoothed yellow tail sushi, there are plenty of swanky places that'll relieve you of £30 for the pleasure.

This is crowd-pleasing stuff. What makes the restaurant (I'm at the newest, in West Hampstead) interesting, is that Bjerrum is committed to using only sustainable fish. On the back of the menu is a list of every type used, where it comes from and how it has been caught. The sardines are from Cornwall, the crab from Devon and the scallops from Mull. I don't usually get a warm, fuzzy feeling from cold, raw fish, but this is to be applauded.

So, to recap – big portions of planet-friendly food at reasonable prices. And the verdict: some of it is tasty and some of it is really not.

While we're putting together our epic order, Mr M and I have a Kirin beer and some lotus-root crisps. These are much less nice than they look, simultaneously stale-tasting and sharp enough to cut the gums. (I think that's the root's fault, not Feng Sushi's.)

The chilli edamame are a bit gloopy, but a hearty amount of heat gets the tastebuds jumping, while chilli sauce makes another appearance with the deep-fried sushi, which is much better than its billing. A bit of a revelation, in fact, although I suspect the calorie count prevents it becoming a regular order. Sushi, by the way, can be ordered with white or brown rice, which gives a waft of health over proceedings, as does that deluxe sashimi, which is beautiful: pearlescent cuts with microherbs, all tasting very fresh and very of itself.

But a fusion salad of chilled ramen noodles with cherry tomatoes, prawns, tamago and wakame with a squirt of creamy dressing is misguided. The noodles, omelette slivers and seaweed work inasmuch as one assumes lots of Japanese people like that combo, but the tomatoes and prawns look, and taste, like somebody said the phrase "classic with a twist" at a menu-planning meeting.

Similarly, an X-ray salad (£7.50) felt like a crisper-drawer clear-out: peppers, pomegranate, edamame, avocado and chives. Bright enough to look at, but a bugger to pick up with chopsticks and nothing much to taste.

By now plates are stacked everywhere on our table, but the best is last. The donburi dish is so good – fresh ginger packing a real punch over the oily fish and soft rice – that the woman at the next table orders it, too, even though she has just paid her bill.

We shouldn't have ordered the chocolate mochi for pudding. Not only because we are full, but because the strange, slightly waxy texture (made of pounded rice) is a bit too peculiar. In Japan, the mochi are filled with red-bean paste. Chocolate is better, but only just.

I'll definitely come back to Feng Sushi – I just reckon it takes a few goes to find out which dishes to avoid, and which always to order (I'm afraid that tempura-battered tuna roll already has me in a vice-like grip).

7/10

Feng Sushi 280 West End Lane, London NW6, tel: 020 7435 1833 Lunch and dinner daily; £50 for two with beer

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