Lightly baste chicken breast with lemon butter; sprinkle with chicken spice mix. Place chicken, basted-side down on a hot grill; cook 1 minute. Rotate chicken 1/4 turn. Baste and season the top of the chicken again; cook 2 to 3 minutes more. Turn over chicken breast; cook 1 minute. Turn chicken 1/4 turn; cook until chicken has cooked through.
Place cooked chicken breast in a baking pan. Top chicken with cooked bacon, mushrooms and cheese. Place under broiler or into a hot oven until the cheese melts. Place the chicken on the bottom half of a toasted Kaiser bun. Top with lettuce and tomato.
