Preheat oven to 400˚. Line a baking sheet with parchment paper. Lay sliced zucchini in a single layer and on other side of baking sheet, place the sliced onion. Drizzle everything with olive oil and season with salt and pepper. Bake for approximately 20 minutes, or until vegetables have a little color. Remove from oven and let cool.
While vegetables are baking, combine both cheeses, tomatoes, garlic paste, Italian seasoning and salt and pepper in a small mixing bowl; set aside.
Heat a nonstick or cast-iron pan over medium heat. To assemble sandwich, spread half the cheese mixture on one side of bread, top with roasted vegetables, then top with remaining cheese mixture and other slice of bread. Spread top of bread with softened butter and place buttered side down in heated pan. Cook approximately 3-4 minutes or until lightly browned. If necessary, turn heat down so the bread doesn’t burn. While the sandwich is cooking, spread remaining softened butter on top of bread. Flip sandwich over and cook another 3-4 minutes or until lightly browned.