1Heat the oven to 375°F and arrange a rack in the middle.
Spread out the mushrooms in a single layer in a baking dish. Add the oil, garlic, salt, pepper to taste, and the thyme and parsley and toss to combine. Roast the mushrooms until soft and a little browned, about 25 minutes. Set aside, keeping them warm.
Melt the butter over medium heat in a skillet or nonstick frying pan large enough to hold 4 slices of brioche. Distribute the cheese evenly over the bread slices. Cook until the underside of the brioche is golden and crisp, about 4 minutes.
Distribute the roasted mushrooms between 2 of the brioche slices. Top with the remaining 2 slices, cheese-side down. Cut the sandwiches diagonally in half, transfer to warmed plates, and serve.