Yellow Squash and Bell Pepper Torte

Name: Yellow Squash and Bell Pepper Torte
Ingredients: 4 yellow bell peppers, halved lengthwise, stemmed, and seeded; 5 to 6 tablespoons olive oil, divided; 6 garlic cloves, thinly sliced; 2 pounds yellow squash (4 medium), sliced diagonally 1/2 inch thick, divided; 1 pound red onions (2 medium), cut crosswise into 1/3-inch-thick rounds; 3 tablespoons red-wine vinegar; 1/2 cup grated Parmigiano-Reggiano; 1/3 cup chopped basil; Equipment: an 8- to 9-inch springform pan; Garnish: Parmigiano-Reggiano shavings and basil leaves