To prepare salmon, rinse with cold water. Pat dry with paper towels, and set aside. Combine lemon juice, white wine, olive oil, capers and quarter 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, quarter 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 13 cm 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. ~-