"Had to in some way invent a meatloaf with absolutely no ketchup or tomatoes. My fiance absolutely loves cheese, so I embellished and also took from other recipes here and there. Trying this, I believe you will love it. It has been a success. Good with red wine and twice-baked potatoes for homecooked TLC."
Mix ground beef, red onion, bread crumbs, eggs, salt, and pepper together in a large bowl. Divide mixture in half. Press half of the mixture in the bottom of a loaf pan.
Run 2 fingers lengthwise down the center of the beef mixture. Fill hollowed space with mozzarella cheese. Mold remaining beef mixture around cheese, pressing down firmly.
Heat oil in a large skillet over medium-high heat. Cook bacon slices until lightly browned, about 5 minutes. Arrange bacon slices over beef mixture.
Cook and stir 3/4 teaspoon mustard and 3/4 teaspoon Worcestershire sauce in the bacon grease in the skillet for 1 minute. Add bell pepper; cook until softened, 3 to 4 minutes. Spread bell pepper over bacon.
Place remaining mustard, Worcestershire sauce, and mushrooms in the skillet; cook and stir until soft, about 5 minutes. Spread evenly over bell pepper. Scatter feta cheese over mushrooms.
Cover loaf pan with aluminum foil; place on top of a baking sheet.
Bake in the preheated oven until an instant-read thermometer inserted into the beef reads at least 160 degrees F (71 degrees C), 60 to 90 minutes.