Open-Faced Roast Beef Sandwiches with Braised Cabbage Slaw & Russian Dressing

1 tablespoon olive oil 1 cup thinly sliced onion 1 1/2 cups thinly sliced peeled Fuji apple 5 cups thinly sliced green cabbage 1/4 cup water 2 teaspoons white wine vinegar 1/4 teaspoon caraway seeds, crushed 1/4 teaspoon salt 2 tablespoons chopped fresh parsley 2 tablespoons 2% low-fat Greek-style plain yogurt 1 1/2 tablespoons canola mayonnaise 1 tablespoon prepared horseradish 1 tablespoon ketchup 1 teaspoon stone-ground mustard 1/2 teaspoon freshly ground black pepper 1/2 teaspoon fresh lemon juice 1/2 teaspoon Worcestershire sauce 1/4 teaspoon salt 1/4 teaspoon sweet paprika Dash of ground red pepper 1 pound thinly sliced low-sodium deli roast beef 6 (1-ounce) slices rye bread, lightly toasted