How to Make It
Prepare the flounder: Score flesh of whole flounder on both sides in a cross-hatch pattern. Remove head and place fish in a large shallow dish. Pour buttermilk over fish. Cover and refrigerate 1 hour, turning occasionally.
Prepare the mojo verde: Place garlic, cilantro, parsley, vinegar, salt, pepper, and cumin in a food processor. Add olive oil, and pulse until combined, 6 or 7 times. Transfer to a bowl, and set aside.
Pour vegetable oil to a depth of 3 inches in a large stockpot. Heat to 350°F over medium-high. Place masa in a shallow dish. Remove fish from buttermilk, and add to masa. Press masa onto fish, and let stand to hydrate, 5 to 10 minutes. Shake off excess masa. Slowly submerge fish in oil, and fry for 5 minutes.
Remove fish from oil, and place on paper towels to drain. Carefully transfer fish to a platter. Season with salt, and drizzle with mojo verde. Squeeze lemon over fish, and serve immediately with Chile Mayo.