Little Italy Sausage Sandwiches

1 pound sweet Italian sausage links, cut in half lengthwise
1 cup sliced onions
1 (6 ounce) can Italian tomato paste
1 cup water
2 cups mixed salad greens
Italian salad dressing, to taste
4 (6- to 7-inch) French rolls, hollowed out to leave
    a 1/2-inch shell

Brown sausage with onions; drain. Add tomato paste and water, mixing well. Bring to a boil; reduce heat. Simmer for 10 minutes.