Trim the tough woody ends from the asparagus and cut the spears into 2-inch pieces. Slice the mushrooms into 1/4-inch thick pieces. Having both vegetables prepped and ready will allow you to work efficiently once you start cooking.

Melt 1 tablespoon of butter in a large skillet over medium-high heat. Add the sliced mushrooms and sauté for about 5 minutes, stirring occasionally, until they release their moisture and begin to lightly brown. This develops their savory flavor before they join the gratin. Set the cooked mushrooms aside on a plate.

Melt 1/4 cup butter in a saucepan over medium heat. Once melted, whisk in the flour to create a paste, stirring constantly for about 1 minute to cook out the raw flour taste. Gradually pour in the milk while whisking to prevent lumps from forming. Continue cooking and stirring until the sauce thickens, about 3-4 minutes. The sauce should coat the back of a spoon when ready. Remove from heat and stir in the dry sherry, mozzarella cheese, salt, pepper, and garlic powder until the cheese melts completely and the sauce is smooth.

While the sauce finishes cooking, preheat your oven to 350°F and lightly butter a 9x13-inch baking dish (or similar size). Once the oven is ready, spread the prepared asparagus pieces evenly across the bottom of the dish. I find that spreading the asparagus in a relatively even layer helps it cook more uniformly. Scatter the sautéed mushrooms from Step 2 over the asparagus.

Pour the béchamel sauce from Step 3 evenly over the asparagus and mushroom mixture, making sure to coat the vegetables well. Bake in the preheated 350°F oven for 30 minutes, until the sauce is bubbling around the edges and the top is lightly golden. The gratin is ready when the vegetables are tender and the sauce has reduced slightly into a creamy coating.