Greek Cheeseburgers

4 ounce(s) crumbled feta cheese

1 teaspoon(s) ground cumin

1 teaspoon(s) dried mint

1/2 teaspoon(s) salt

1/2 teaspoon(s) coarsely ground black pepper

2 green onions , thinly sliced

1/4 English (seedless) cucumber , unpeeled and cut into 1/4-inch dice

1 clove(s) garlic , crushed with garlic press

1 container(s) (8 ounces) plain low-fat yogurt

4 hamburger buns

4 Romaine lettuce leaves

1 large tomato , thinly sliced