Preheat grill on high. Clean mushrooms and remove stems and gills. Set aside.
Form Italian sausage into four patties. Place cherry tomatoes and sea salt into blender and blend until chunky.
Lower grill heat to medium and place mushrooms cap side down. Spoon about ¼ cup of sauce onto each mushroom. Place sausage patties on grill. Cook sausage and mushrooms, about 8 minutes each. Turn sausage patties over and layer five pieces of pepperoni and two slices mozzarella on each patty. Cook 5-8 minutes, until burgers are cooked through and cheese is melty.
Remove mushrooms from grill. Stack sausage burgers on top of mushroom caps and garnish with fresh basil.