Grill or broil tuna 4 inches from heat source until fish just begins to flake, about 7 to 9 minutes, turning once. Remove and discard skin. Cut tuna into chunks; set aside. Cook pasta according to package directions, omitting salt. Drain; set aside. Spray large nonstick skillet with cooking spray. Add bell peppers, onion and fennel seeds; cook over medium heat until crisp-tender. Whisk together milk, flour, mustard and black pepper in small bowl until smooth; add to skillet. Cook until thickened, stirring constantly. Stir in tuna and pasta; reduce heat to low and simmer until heated through, about 3 minutes. Serve immediately.