Preheat oven to 425 degrees. Lightly spray baking sheet with olive oil.
For the mozzarella bites: Warm a small skillet over medium heat. Toast panko bread crumbs until golden brown, about 1-2 minutes, stirring constantly. Remove from pan and transfer to a bowl. Add Parmesan cheese to bowl and mix.
Crack egg into a small bowl and whisk. Cut mozzarella sticks into 1-inch pieces (about 3 pieces per cheese stick).
Dip the cheese in the egg to coat completely, letting the excess egg drip back into the bowl. Then thoroughly coat the cheese in the bread crumb mixture, patting to adhere completely. Repeat process with the rest of the cheese. Line the cheese bites evenly on the baking sheet.
Bake 2-3 minutes, until cheese softens, but isn't oozing onto the pan. Remove from oven, then immediately transfer cheese bites to a plate, using a spatula. (This is to prevent the bites from losing their shape, and from the cheese completely melting).
For the marinara sauce: Heat a small skillet over medium heat, then add olive oil. Once olive oil is hot, add diced onion and cook until translucent and aromatic, about 3-4 minutes.
Pour crushed tomatoes, tomato paste, parsley, oregano, garlic, salt and pepper in large pot. Add cooked onions. Bring mixture to a boil, then reduce to a simmer and cook until flavors blend, about 20-30 minutes, stirring occasionally. Remove from heat and serve with mozzarella bites.
Recipe by sweet caroline's cooking at https://sweetcarolinescooking.com/2013/04/09/lighten-up-baked-mozzarella-bites-w-homemade-marinara-sauce/