Preheat oven to 400° F. In a small bowl, combine bread crumbs and milk to soak. Set aside.
Heat oil in a non-stick pan or skillet to medium heat. Add mushrooms, celery, and onion. Cook until fragrant and soft, about 6-8 minutes, stirring occasionally. Stir in garlic and cook 1-2 minutes more. Remove from heat.
Stir in seasoning and tomato paste. Let cool slightly.
In a large bowl, mix together beef, sausage, bread crumbs, eggs, Worcestershire sauce, salt, and pepper until well incorporated. Mix in cooked veggies until well incorporated.
Place meat mixture into two greased 4x9 bread loaf pans.
Cover with foil and bake 20 minutes. Remove foil and bake 20 to 25 minutes more, or until internal temp reaches 160 degrees (see notes below about the safe internal temp).
Coat tops with sauce and broil 2-3 minutes. Remove from oven and cool on a wire rack for 10 to 15 minutes. Cut into slices and serve.
The safe internal temp of ground beef is 165 degrees F when tested with a thermometer. Bake meatloaf to 160, then broil with sauce for 2 to 3 minutes. It will also continue to cook a few degrees while resting.
Don't have a bread loaf pan? Form meatloaf mxiture into rectangular mounds on a foil or parchment-lined baking sheet.
Breadcrumbs and milk: Breadcrumbs creates extra space and air between the meat proteins, it also holds some of the fat from the meat.
Mix in eggs: Adding eggs helps hold everything together.