Portobello Burgers with Red Peppers and Gorgonzola

Name: Portobello Burgers with Red Peppers and Gorgonzola
Ingredients: 4 large portobello mushrooms, stems removed; ¼ cup balsamic vinegar; 3 garlic cloves, minced, plus 1 whole clove, peeled and halved; 1 tablespoon chopped fresh thyme; ¼ cup extra virgin olive oil, plus more for brushing; 2 small red bell peppers; Kosher salt and freshly ground black pepper to taste; ½ cup crumbled Gorgonzola cheese; Eight ½-inch-thick slices rustic country bread, cut from a 6-inch-high loaf; ½ cup mayonnaise; ½ cup shredded fresh basil