The Sandwich Ratatouille is built around a cooked vegetable stew rather than a single filling, and the fact that the filling is already a finished dish decides how it behaves. Ratatouille is the Provençal stew of aubergine, courgette, sweet pepper, tomato, and onion cooked down slowly in olive oil with garlic and herbs until each vegetable softens and the whole thing slumps into a loose, glossy mass. It is not a raw garnish and not a salad: it is a stew, spoonable, oil-rich, and assertively savory, packed cold or barely warm into a split crusted loaf or a length of baguette, usually with little else so the vegetables stay in front.
The logic follows from the oil and the moisture. Ratatouille carries a lot of olive oil and tomato liquid, so the sandwich works the way a Pan Bagnat works: given a few minutes the bread drinks the oil, the crumb softens into the stew, and the loaf and the filling stop being separate things. That same wetness is the constraint. A soft loaf with a thin crust dissolves into mush, so the bread has to have a real crust to hold its shape while the interior takes on the oil. The stew is intensely savory and a little sweet from the cooked pepper and tomato, which means the build stays spare; a slice of fresh goat cheese pressed in is the one common addition, its lactic tang cutting the richness the way it does on a Provençal plate. It eats cool or barely warm, never hot, and is one of the few sandwiches that improves with a short wait rather than degrading.
Variations stay close to the Provençal vegetable shelf. A disc of soft goat cheese or a smear of brousse turns it tangier and rounder; a few torn basil leaves lift it; a hard-boiled egg gives it body for a fuller lunch. Each holds the stew as the fixed point and adjusts only what sharpens or steadies it. The Sandwich Ratatouille belongs with the casserole-into-bread tradition the catalog groups under Plat-en-Sandwich, the dishes that began as a hot plate of food and were later folded into a loaf. Its specific contribution is a finished oil-rich vegetable stew that the bread is meant to absorb, so the sandwich gets better while it sits rather than worse.