In a skillet over medium heat, melt butter and add sliced onions and salt. Cook until translucent and tender, stirring ocassionaly, about 10 minutes.
Add sliced mushrooms and cook for another 10 minutes, until the mixture has cooked down to 1.5-2 cups in volume and onions and mushrooms are dark golden brown. Set aside.
Preheat the oven to 350℉.
For the Meat:
In a mixing bowl combine ground beef, breadcrumbs, eggs, worcestershire sauce, garlic powder, beef bouillon, pepper, and thyme. Mix well until combined.
Line a 9x5 bread pan with tinfoil and spray with baking spray. Layer about ⅔ of the meat mixture in the bottom of the pan and spread up the sides to create a meat boat.
Layer ¾ cup mozzarella cheese in the hollow of the meat, then add the caramelized onion and mushroom filling. Layer another ¾ cup of cheese on top, then cover with the last ⅓ of the meat mixture and spread evenly to cover the filling.
Sprinkle ½ cup of mozzarella cheese over the top of the meatloaf. Bake for 50-60 minutes, until beef reaches internal temp of 160℉.
Carefully drain any grease from edges if necessary. Slice and serve while warm. Enjoy!
Notes
Once cooled, store leftovers in an airtight container and refrigerate for up to 3 days, or freeze for up to 3 months.