Preheat the broiler of your oven.

Place the bread slices on a large baking sheet. Broil until toasted and lightly browned on top, then flip the bread over and broil the other side, checking every 2 minutes or so to make sure it doesn't burn. (It can happen fast!)

In a large skillet over medium heat, warm up the olive oil until it's shimmering. Add the shallots and cook for 4 minutes, stirring often, until softened and just beginning to brown.

Increase the heat to medium-high and stir in the mushrooms; cook for about 5 minutes, or until the mushrooms release their juices and the liquid cooks off.

Add the asparagus, thyme and vinegar. Cook for 2-3 minutes, or until the asparagus rounds are tender-crisp. Season to taste with salt and pepper.

Divide the asparagus mixture onto the bread. Sprinkle the cheese over the veggies, then return the baking sheet to the oven and broil until the cheese has melted and is beginning to brown in places (checking every 2 minutes to make sure they don't burn). Garnish with additional thyme and serve.