Warm Cheese & Mushroom Toasts

• 2 Tbs. unsalted butter
• 1 large shallot, finely diced (about 1/4 cup)
• Kosher salt
• 10 oz. mixed mushrooms (like shiitake, cremini, and oyster), trimmed and thinly sliced
• Sixteen 1/2-inch slices baguette, cut on a sharp diagonal so they're about 3 inches long
• 1-1/2 cups grated Gruyère (about 4 oz.)

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.