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1
Preheat broiler.
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2
Broil tomatoes, onion, and garlic in a foil-lined shallow baking pan 2 to 3 inches from heat, turning frequently, until lightly charred on all sides, about 5 minutes for garlic, 12 to 15 minutes for onion and tomatoes (flesh should be soft).
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3
Alternatively, char vegetables in a hot well-seasoned griddle or cast-iron skillet over moderate heat.
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4
Core tomatoes and peel garlic, then puree both with onion and salt in a blender until smooth (use caution when blending hot ingredients).
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5
Force through a large sieve into a bowl, discarding solids.
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6
Heat remaining 2 tablespoons oil in a wide 4-quart heavy pot over moderately high heat until just hot.
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7
Stir about 1/2 cup tomato sauce into oil (this reduces spattering) and, when bubbling vigorously, add remaining sauce and basil.
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8
Fry sauce, stirring occasionally, until thickened, about 10 minutes.
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9
Add grapes, plantains, and mangoes to sauce and simmer gently, uncovered, stirring occasionally, until sauce is thickened and fruit is tender but still intact, about 30 minutes.
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10
Season sauce with salt.
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11
While sauce is simmering, preheat oven to 350F.
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12
Pour sauce over stuffed chiles and bake, covered with foil, until cheese is completely melted and sauce is bubbling, about 30 minutes.
