Preheat the oven to 375 degrees F. Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the sliced mushrooms and half the minced garlic. Cook until the mushrooms release their liquid and turn golden, about 6 minutes. Set the mushrooms aside to cool slightly.

In a large bowl, combine the ground beef, breadcrumbs, egg, remaining onion and garlic, Worcestershire sauce, salt, and black pepper. Mix with your hands until just combined.

Place half the meat mixture into a loaf pan and press it into an even layer. Lay 3 slices of provolone cheese over the meat, then spread the cooked mushrooms on top. Add the remaining 3 slices of provolone.

Cover the filling with the rest of the meat mixture. Press the edges together to seal the cheese and mushrooms inside completely.

Bake for 45 to 50 minutes until the internal temperature reaches 160 degrees F. Let the meatloaf rest for 10 minutes before slicing.