Swiss Steak

1 tablespoon margarine
1 (2 pound) chuck roast, cut 1-inch thick
1 envelope onion soup mix
1 can mushrooms, drained
1/2 medium green bell pepper, sliced
1 (20 ounce) can tomatoes, drained and
    chopped, reserving juice
Salt and pepper, to taste
1 tablespoon A-1