Heat olive oil in a large skillet over medium-high heat. Add mushrooms and cook until liquid has evaporated, 7 to 10 minutes. Add parsley, thyme, and mint; toss together. Season well with salt and pepper. Remove from heat. Let cool, then add the cheese. Gently mix together.
Preheat a broiler. Rub each side of the toast lightly with garlic. Distribute mushrooms and cheese on top of toast. Arrange the toasts on a baking sheet in a single layer and place under the broiler until the cheese melts. Place on a platter and drizzle with lemon juice. Serve immediately, garnished with parsley leaves.