Portobello Mushroom Burger With Bruschetta Topping

8 roma (plum) tomatoes, diced

⅓ cup chopped fresh basil

¼ cup shredded Parmesan cheese

1 tablespoon balsamic vinegar

2 cloves garlic, minced

1 teaspoon olive oil

½ teaspoon kosher salt

½ teaspoon ground black pepper

2 large portobello mushroom caps, stems removed

2 tablespoons shredded horseradish Cheddar cheese, or to taste

2 kaiser rolls, split

Mix roma tomatoes, basil, Parmesan cheese, balsamic vinegar, garlic, olive oil, kosher salt, and black pepper in a bowl and refrigerate to blend flavors, 1 to 2 hours.

Preheat grill for medium heat and lightly oil the grate.

Grill portobello mushrooms with gill sides up on an upper rack of the grill until juicy and hot, about 15 minutes. Spoon tomato mixture into mushroom caps to cover entire cap. Continue grilling until topping is heated through, another 15 to 20 minutes. Top with horseradish Cheddar cheese if desired; grill until cheese has melted, 1 to 2 more minutes. Serve on kaiser rolls.