Nonstick vegetable oil spray
1 large onion, thinly sliced
2 pounds Roma tomatoes, halved lengthwise
1 pound carrots, peeled, cut into 1/2-inch-thick rounds
2 garlic cloves, unpeeled
1 tablespoon olive oil
2 1/2 cups water
2 3/4 cups (about) low-fat (1%) milk
1/2 cup thinly sliced fresh basil