Rinse fish and pat dry; cut into serving-size pieces. Rub fish with 2 teaspoons oil. Place fish pieces slightly apart in a single layer in a 10- by 15-inch baking pan. Bake in a 425° oven, uncovered, until fish is barely opaque in thickest part (cut to test), 10 to 15 minutes.
In a 10- to 12-inch frying pan, combine remaining oil, onion, garlic, and chili. Stir often over medium-high heat, until onion is lightly browned, about 5 minutes. Meanwhile, drain tomatoes, reserving juice. Mix reserved tomato juice, lemon juice, and cornstarch. Add tomatoes and juice mixture to onion; stir until mixture boils and thickens.
With a slotted spatula, transfer fish to plates. Spoon sauce over fish; sprinkle with cilantro. Add salt and pepper to taste.