Preheat oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, egg, milk, minced onion, minced garlic, thyme, salt, and pepper.
Mix ingredients by hand until just combined; do not overwork the meat.
Shape the mixture into a 9x5 inch loaf and place on a parchment-lined baking sheet or in a loaf pan.
Bake for 45 to 55 minutes, or until the internal temperature reaches 160°F (71°C).
While meatloaf bakes, melt butter in a skillet over medium-high heat.
Add sliced mushrooms and sauté until browned and liquid has evaporated, approximately 8 minutes.
Whisk in flour and cook for 1-2 minutes to create a light roux.
Slowly whisk in beef stock and Worcestershire sauce, simmering until the gravy thickens.
Remove meatloaf from oven and let rest for 10 minutes before slicing.
Serve slices topped generously with the mushroom gravy.