Melt the butter in a large heavy-duty skillet over medium heat. Add the shallot, season with 1/2 tsp. salt, and cook, stirring, until softened, about 3 minutes. Raise the heat to high, add the mushrooms and another 1/2 tsp. salt, and cook, stirring frequently, until softened and browned, 3 to 5 minutes longer.
Set an oven rack about 8 inches away from the broiler element and heat the broiler to high. Arrange the bread slices on a large rimmed baking sheet. Broil until lightly toasted, 2 to 4 minutes, rotating the pan as needed for even toasting. Flip the bread, mound a heaping tablespoon of the mushrooms on each piece, and sprinkle with the Gruyère. Broil until the cheese is melted and lightly browned, 4 to 7 minutes. Serve immediately.
Make Ahead Tips
You can sauté the mushrooms, cool to room temperature and refrigerate, tightly wrapped, for up to 1 day. Bring to room temperature before using.
nutrition information (per serving):
Photo: Scott Phillips