In a large bowl, combine ground meat, Parmesan cheese, oregano, lemon juice, teriyaki sauce, and garlic salt.
Divide mixture in half. Shape each half into a ball; pat each ball onto waxed paper to form a flat burger that's 8 inches in diameter.
Spread tomato paste in the center of one circle of meat. Add mozzarella cheese and mushrooms. Top with second circle of meat. Seal edge.
Grill on the rack of an uncovered grill directly over medium* coals for 30 to 35 minutes or until done (an instant-read thermometer inserted in side of burger registers 160 degree F), using a wide spatula to turn the burger once halfway through grilling time. Cut into wedges to serve. Makes 6 servings.
To test for medium heat, you should be able to hold your hand over the coals at the height of the food for 4 seconds before you have to pull away.