For a better hotdish, we like to use homemade ingredients. (Except the tater tots! We'll always have a soft spot for the frozen ones.)
Adjust oven rack to center position and preheat oven to 350°F . In a large bowl, toss mushrooms with olive oil and 6 sprigs thyme, then season with kosher salt and freshly ground black pepper. Spread seasoned mushrooms on a foil-lined rimmed baking sheet. Bake until deeply browned, about 45 minutes. Discard thyme sprigs and increase oven temperature to 375°F .In a medium saucepot, combine roasted mushrooms with 6 remaining thyme sprigs, milk, onion, and garlic and bring to a boil.
Heat, stirring, until sauce comes to a simmer and begins to thicken slightly, about 10 minutes. Reduce heat to low and cook, stirring, until sauce is thick enough to coat the back of a wooden spoon, about 3 minutes. Season to taste with kosher salt and freshly ground black pepper.In a large skillet, heat oil over medium-high heat. Add ground beef and break up into crumbles with a whisk or potato masher. Cook until well browned, 5 to 10 minutes.