Wash and lightly dry fish. Chop into 3 to 4 ounce pieces and massage 2 teaspoons of oil onto surface. Arrange fish a few inches apart in one layer in a 10 by 15 inch baking dish. Bake 10 to 15 minutes (fish will no longer be translucent).
Add the rest of the oil, onion, garlic, and chili to a skillet and sauté over medium-high heat 5 minutes or until onion is golden.
In the meantime, strain tomatoes in a colander, saving the juice. Whisk together saved tomato juice, lemon juice, and cornstarch. Stir in tomatoes and juice blend to skillet and let boil, sauce will become thick.
Move fish to serving platter and drizzle sauce over fish. Season with cilantro, salt, and pepper.