Open sausage casings using kitchen shears; then just squeeze the sausage into the pan for browning.
Cook pasta in boiling salted water in a large Dutch oven according to package directions. Drain and return to pan.
Meanwhile, brown sausage, bell pepper mix, and garlic in oil in a large nonstick skillet over medium-high heat, stirring often, 8 to 10 minutes or until meat crumbles and is no longer pink. Drain. Stir meat mixture, pasta sauce, and Cheddar cheese into pasta. Spoon half of pasta mixture into a lightly greased 5-qt. slow cooker coated with cooking spray.
Combine mozzarella cheese and fontina cheese. Sprinkle half of mozzarella mixture over pasta mixture in slow cooker. Top with remaining pasta mixture, remaining mozzarella mixture, and Parmesan cheese. Cover and cook on Low 3 hours. Let stand, covered, 10 minutes before serving.
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