Juicy Portobello Mushroom Burgers with Pesto

Name: Juicy Portobello Mushroom Burgers with Pesto
Ingredients: 2 cloves garlic; 4 large portobello mushrooms (about 3 ounces each and 4 inches wide); 2 tablespoons balsamic vinegar; 1 tablespoon low-sodium tamari or soy sauce; 1 tablespoon olive oil; 1/2 teaspoon kosher salt; 1/4 teaspoon freshly ground black pepper; 1 small red onion; 4 ounces roasted red peppers (about 3/4 cup); Vegetable oil; 4 hamburger buns; 1/2 cup vegan or regular pesto sauce ( homemade or store-bought ); 1 cup packed baby arugula (about 1 ounce)