Preheat the oven to 375°.
In a food processor, add the onion, carrot, celery, mushrooms and bell pepper and pulse on high until it is finely minced.
Add the processed mixture to a bowl along with the ground beef, eggs, cheeses, bread crumbs, and salt and pepper and mix thoroughly until combined.
Transfer the mixture to an 8 x 4 loaf pan and pack it in.
Bake at 375° for 45 minutes or until cooked throughout.