Here is a vegetable sandwich whose centre tastes of savour rather than freshness, and that inversion is exactly what makes it hard to build. Cooked mushrooms bring depth, a near-meaty umami, an earthy register no salad vegetable reaches. They also bring water, and a great deal of it. The defining element is the mushroom cooked hard: sliced, set in a hot dry pan or with very little fat, and held there until it has thrown off its liquid, browned, and concentrated into something dense. Only then does it go into a split crusted loaf, finished with salt and often a little chopped parsley or shallot.
The craft is entirely about that water. A mushroom is mostly liquid, and a mushroom that has not been cooked down will release it into the crumb and leave the sandwich sodden within minutes. So the technique runs opposite to most vegetable builds: not preserving moisture but driving it off, cooking past the point where the pan looks wet until the slices are reduced and glossy and the flavour has stepped up to match. The reward is a filling with real backbone, savoury enough to carry a sandwich with almost nothing else. The bread needs a genuine crust to resist what moisture remains, and the mushrooms should go in just warm, not hot, so they steam the crumb as little as possible. Eaten within a few minutes of assembly it holds; left to sit it is the build that fails fastest.
Variations stay in the same earthy register. The same loaf takes the cooked mushroom over a thin smear of crème fraîche and shallot, the dairy rounding the savour into something closer to cold-weather food; or a stronger wild mushroom in season for more depth and less water; or a few shavings of a hard cheese melted lightly over the warm slices for a salt-and-nut edge. Each holds the bread constant and adjusts only what sits with the mushroom. The Sandwich aux Champignons belongs with the plant-forward builds the catalog groups under Sandwich Végétarien. Its specific contribution to that shelf is umami from a vegetable, won only by cooking the water out first.