1/4cupchopped fresh basilplus additional for garnish
Position a rack in the upper third of your oven and preheat to broil. Lightly coat a 2-quart, broiler-safe baking dish with nonstick spray.
In a small bowl, stir together the panko, Parmesan, and lemon zest. Melt 2 tablespoons of the butter, pour over the top, then stir to moisten. Set aside.
Meanwhile, heat the oil and remaining 1 tablespoon butter in a large, wide saucepan or Dutch oven over medium heat. Add the asparagus, onion, and mushrooms. Cook, stirring frequently, until the onion is soft and golden and the mushrooms brown and have given up their liquid, about 15 minutes.
Sprinkle the flour over the top. Cook, stirring, for 1 to 2 minutes more, until all of the flour turns golden and no white bits remain.
Slowly add the milk a few splashes at a time, stirring between each addition to prevent lumps from forming. Increase the heat to medium high.
Stir in the salt, pepper, dried basil, and nutmeg. Cook and stir, allowing the sauce to bubble, running a wooden spoon or rubber spatula along the bottom of the pan to prevent sticking.
Continue cooking and stirring until the sauce reduces and thickens to resemble a creamy gravy, about 8 to 10 minutes. Run the spoon or spatula along the bottom very frequently at the end to make sure that your sauce isn’t sticking.
Remove the sauce from the heat. Stir in the Greek yogurt, 1 tablespoon lemon juice, and fresh basil. Taste and adjust the seasoning as desired.
Spoon the mixture into the prepared baking dish and spread into an even layer.
Sprinkle the breadcrumb mixture over the top.
Transfer to the oven and broil, watching closely, until the casserole is bubbling and beginning to brown on top, 1 to 3 minutes, depending on your broiler. Let stand for 5 to 10 minutes prior to serving. Garnish with additional fresh basil.
TO MAKE AHEAD: The casserole can be assembled without the topping through Step 9 up to 1 day in advance or frozen up to 1 month in advance; be sure you are using a freezer-safe baking dish. Cover tightly and refrigerate. When ready to bake, let come to room temperature (if the dish is in the freezer, place it in the refrigerator the night before to thaw). Sprinkle with breadcrumb topping and bake as directed.
TO STORE: Refrigerate casserole in an airtight storage container for up to 4 days.
TO REHEAT: Rewarm leftovers in a baking dish in the oven at 350 degrees F.
TO FREEZE: Freeze leftover casserole in an airtight, freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.