In a medium bowl, combine ground meats, 1½ teaspoons of salt and ½ teaspoon of pepper. Shape mixture into patties, about 3 inches in diameter and 1-inch thick. Press an indentation into the center of each patty.

In a large sauté pan set over medium-high, heat 1 tablespoon of olive oil. Add patties and cook, covered, for 6-8 minutes per side or until an instant-read thermometer inserted into the side of the burger reads 160°F. Transfer to a plate and cover to keep warm.

Meanwhile, generously brush mushrooms with olive oil and season with salt and pepper to taste. In a large sauté pan set over medium-high, cook in batches for 2-3 minutes each side or until golden and tender. Sprinkle half of the cooked mushrooms with sesame seeds. These will be the “tops”.

Place half of the cooked mushrooms top-side down. Add beef patties and desired toppings. Sandwich with remaining mushrooms, top-side up, to serve.