Preheat a grill pan to medium-high heat. Combine the chopped mushrooms, ground beef, ground pork, garlic, onion, Montreal seasoning, salt, pepper, and beaten egg.
Mix gently and form into 4-6 patties. Brush portobello caps with olive oil. Season with salt and pepper.
Grill burgers for about 4 minutes per side for medium burgers. Grill portobello mushrooms until tender and golden or to your liking.