Mix together olive oil, 4 tablespoons lemon juice, slivered garlic, quarter cup cho pped cilantro, salt and pepper. Add fish fillets and marinate for at least one hour or as long as overnight. To prepare garlic beurre rouge: In a frying pan over medium heat, melt two tabl espoons butter and saute onion, chilies, and minced garlic until soft, stirring often. Add tomatoes and remaining lemon juice. Cook for 10 minutes, stirring o ccasionally. Remove from heat and season to taste. Stir in quarter cup chopped cila ntro. Slowly stir in remaining butter until melted. Drain marinade from fish. Barbecue fillets over low glowing coals (or broil if desired), turning only once. Be careful not to overcook. Remove to a warm servi ng platter and top with garlic beurre rouge. Garnish with lemon slices and cila ntro sprigs.