Question setter's note: [a coulibiac is] usually made with salmon or sturgeon combined with buckwheat, hard–boiled eggs, mushrooms, scallions, wine, herbs, and spices, and served in a brioche or puff pastry.
Web Editor adds (courtesy of Wikipedia): "A coulibiac (from Russian: kulebyáka) is a type of Russian pirog ... In the early part of the 20th century, Auguste Escoffier, the famed French chef, brought it to France and included recipes for it in his masterwork The Complete Guide to the Art of Modern Cookery."
On its Pirog page, Wikipedia explains: "Pirog is a baked case of dough with either sweet or savory filling. The dish is common in Eastern European cuisines. Pirogi are characterised as 'ubiquitous in Russian life' and 'the most popular and important dish' and 'truly national goods' of Russian cuisine."
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