Bake muffin mix according to package directions in an 8" square pan. Cool and cut into cubes. Place cubes on a baking sheet; bake at 250 degrees for 1 hour. Combine lettuce, celery, tomato, green pepper and onion; toss well. Combine olive oil, lemon juice, dry mustard, salt and mayonnaise. Mix with vegetables. Serve with cornbread croutons. Yield: 8 servings.