Heat olive oil in a large skillet until shimmering.
Add onion and saute until translucent, about 5 minutes.
Add garlic and ground beef, season with salt and pepper.
Brown the ground beef until no longer pink.
Add pasta sauce to pan and simmer for five minutes.
Meanwhile, in a large mixing bowl, combine 3 cups of mozzarella cheese, ricotta, Parmesan, eggs, Italian seasoning, parsley, salt and pepper (to taste) and mix well.
Spray the slow cooker with nonstick cooking spray.
Spread a thin layer of the sauce mixture in the bottom of the slow cooker and top with one layer of lasagna noodles; You should be able to get 4 noodles broken in half across.
Spread a layer of the cheese mixture across using a spatula or the back of a large spoon.
Add a layer of the sauce mixture and 9 pieces of pepperoni.
Top with four more noodles.
Repeat until you have four layers, ending with pepperoni on top.
Cover and cook on LOW for 4 hours.
After 4 hours, sprinkle remaining 1 cup mozzarella cheese on top.
Leave uncovered and allow the cheese to melt.
Once cheese has melted, turn off the slow cooker and allow the lasagna to set for about 10 minutes before serving.