Preheat oven to 375°F (190°C) and grease a loaf pan.
Soak breadcrumbs in milk for 2–3 minutes, then add beef, onions, garlic, eggs, and seasonings.
Mix until combined and shape into loaf; bake 45–50 minutes until 160°F internally.
Meanwhile, sauté mushrooms in butter and oil until golden.
Add onions and garlic; cook 2–3 minutes. Sprinkle flour, stir, and gradually whisk in broth.
Add Worcestershire, salt, and pepper; simmer 5–8 minutes until thickened.
Slice meatloaf and spoon mushroom gravy over top before serving.