To prepare salmon, rinse with cold water. Pat dry with paper towels, and set aside. Combine lemon juice, white wine, olive oil, capers and 1/4 teaspoon mustard. Season to taste with salt. Pour over salmon and marinate 30 minutes, turning once. Reserve marinade. Blanch lettuce leaves in boiling water 5 seconds. Drain well. Arrange 2 leaves with stem ends overlapping at center. Spoon about 2 teaspoons marinade over lettuce. Place 1 salmon fillet on lettuce. Top with 1 or 2 more teaspoons marinade, including some of capers. Roll fillet up. Place 2 leaves on top and tuck edges under fish. Bring up bottom leaf edges. Tie package-style with string. Repeat with remaining lettuce, marinade and fillets to make 4 packages in all. Set aside. To make sauce, blend butter and flour in top of double boiler until smooth. Add vinegar, boiling water, bouillon cube half, 1/4 cup mustard, sugar and egg. Cook and stir over hot water until sauce thickens, about 10 minutes. Keep warm. Place salmon bundles on well-greased grill 4 to 5 inches from hot coals and cook 5 to 6 minutes. Turn and cook an additional 5 to 6 minutes. Remove string and serve with mustard sauce. ~-