Stuffed Portobello "Burgers"

Serves:

• 5 oz each), stems removed, caps wiped with a damp paper towel
• 1 large red onion (slices slightly smaller than mushroom caps)
• 6 Tbsp extra-virgin olive oil, plus extra for drizzling
• 1 loaf ciabatta bread, halved horizontally and cut in 6 pieces, or 6 4-in.-square pieces herbed focaccia bread
• 2 Tbsp balsamic vinegar
• 1 clove garlic, crushed through a press
• 1/2 tsp each salt and freshly ground pepper
• 12 oz fontina cheese, cut in 6 slices
• 18 large leaves fresh basil, plus extra for garnish
• 6 thick slices beefsteak tomato