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HEAT oil in large skillet on medium heat. Add mushrooms and peppers; cook and stir 3 min. Remove from heat. Add spinach; stir until wilted. Stir in spaghetti sauce.
MIX ricotta, 1-1/2 cups shredded cheese, 2 Tbsp. Parmesan and egg until blended.
SPOON 1-1/2 cups sauce into slow cooker; top with layers of half each of the noodles (broken to fit) and ricotta mixture. Cover with 2 cups sauce. Top with remaining noodles (broken to fit), ricotta mixture and sauce. Cover with lid.
COOK on LOW 4 to 5 hours (or on HIGH 2 to 3 hours) or until liquid is absorbed. Sprinkle with remaining cheeses; let stand, covered, 10 min. or until shredded cheese is melted.