Swiss Tuna Bunnies

3 ounces Swiss cheese, diced (about 1/2 cup)
1 can tuna, well drained
1/2 cup salad dressing
1 teaspoon lemon juice
2 tablespoons minced onion
1/2 teaspoon salt
4 hamburger buns

Toss all ingredients except hamburger buns lightly until well mixed. Split the buns, spread with butter and then tuna mixture. Wrap individually in foil. Bake about 20 minutes at 300 degrees F to 350 degrees F.