PARIS (Reuters) - Boeuf bourgignon, veal blanquette, duck a l'orange and gratin dauphinois -- all mainstays of French cuisine, and a familiar sight on the menus of bistros and brasseries across the country.

Except these dishes aren't on offer in a quaint eatery with flavours that vary according to the chef's mood -- rather they've been mass produced, vacuum-sealed in congealed 2-kg packs and sold wholesale to restaurants from an icy warehouse, with microwave re-heating instructions stuck on the side.

This is one of France's darker culinary secrets -- that in a country that revels in its gastronomic reputation, anyone, in theory, could open a "traditional" restaurant with little more than a microwave and a grill.

President Nicolas Sarkozy's conservative government has taken the matter in hand. The lower house National Assembly approved a new law this week that will oblige eateries to indicate whether or not their food is freshly cooked or ready-made.

"(Restaurants) are the only place where you really don't know what you're eating," said deputy Fernand Sire of the ruling UMP party, the man behind the original proposal for the law.

"We need to reward professionals who make the effort to cook high-quality produce and make sure the consumer knows he's about to eat something made in the traditional way," he told Reuters.

The rise of "boil-in-the-bag" meals seems something of a contradiction in a country where "the art of the gastronomic meal" was recently included in UNESCO's intangible world heritage list, after years of campaigning by authorities.

But insiders in the catering industry say the phenomenon is more prevalent than customers would like to think.

Roland Heguy, chairman of the French syndicate for the hotel and restaurant industry (UMIH), estimates only 20,000 of France's 120,000 food establishments could actually claim to make all their produce from fresh ingredients.

"MICROWAVE FOR 9 MINUTES"

The food quality bill, which most pass through the Senate in the days ahead before it can become law, is part of a wider package of measures aimed at protecting consumers.

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Benoit, a chef of 15 years who prefers not to give his surname, said the law would be a much-needed wake-up call for a restaurant industry that has been resting on its gilded laurels for far too long.

He recently opened a gourmet restaurant, preparing fresh, quality produce, purchased daily, and says he struggles to compete with rivals who offer cheap lunchtime set-meals comprised of prepackaged, frozen or ready-made goods.

"We've hired a whole team to prepare fresh produce, and they've got just one guy in the kitchen sending out a hundred microwaved ready-meals a day," he told Reuters.

Back in the corrugated metal warehouse outside Paris, the array of foodstuffs is overwhelming and raises uncomfortable questions about many meals enjoyed in bistros or purchased from mouth-watering Parisian delicatessens.

Among the rows of fresh vegetables and exotic spices, are giant freezers full of frozen food, shelves stacked with plastic-wrapped pates and dishes of gourmet salads, bags of pre-fried onions, and 2-kg vats of "beef tongue in spicy sauce" (microwave for 9 minutes).

Those perfectly formed poached eggs you find in brasserie salads? Purchased in buckets of 75, floating in brine, ready to be dipped into hot water for 60 seconds before serving. Cake-bakers, meanwhile, can buy 3-litre cartons of egg white, complete with matching 1-litre cartons of beaten yolk.

"Maybe with this law people will choose to eat in places that actually hire people to prepare fresh food," said Benoit.

The warehouse raises other questions, however, such as where to draw the line between what is fresh and what is pre-prepared, and how strict the law should be with the restaurant trade.

Should a chef really be obliged to tell customers if he's used frozen raspberries in a homemade patisserie, or frozen cepes in a mushroom sauce made from otherwise fresh ingredients?

Roland Heguy warns that the government could go too far in applying the law, and points out that freezing has been used in patisserie for decades, to keep delicate cakes fresh up to the moment they're served.

There is also the question of whether French people care enough about what's on their dinner plates, in a country riddled with contradictions when it comes to the national cuisine.

This year, gourmet frozen food retailer Picard was voted the nation's favourite brand for the second year in a row.

And France is one of the most profitable markets in the world for U.S. burger chain McDonald's.