1. Cook pasta according to package directions, omitting salt and fat. Drain pasta in a colander over a bowl, reserving 3/4 cup pasta water.
2. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add fennel and onion to pan; sauté 3 minutes. Add garlic, pepper, and salt; sauté 2 minutes, stirring constantly. Add reserved pasta water and marinara to pan; cook 1 minute. Add mussels; cover and cook 3 minutes or until shells open. Discard any unopened shells. Place 1 cup pasta in each of 4 shallow bowls. Top each serving with about 1 1/2 cups mussels mixture. Sprinkle evenly with parsley.