Portobello Fresca Melts

Name: Portobello Fresca Melts
Ingredients: ¼ cups Balsamic Vinegar Plus Extra For Serving; 2 Tablespoons Tamari (for Gluten Free) Or Soy Sauce; 3 Tablespoons Plus 1 Teaspoon Olive Oil; 2 Tablespoons Minced Fresh Rosemary (or 2 Teaspoons Dried); 1 Tablespoon Grill Seasoning; 2 Tablespoons Minced Red Onion; 2 cloves Garlic, Peeled And Smashed; 6 whole Portobello Mushroom Caps; 2 pints Grape Tomatoes (2 Pints Is About 6 Cups); 6 cups Lightly Packed Baby Or Regular Spinach (more Or Less As Desired); 4 ounces, weight Grated Fontina, Swiss, Or Provolone Cheese