Drizzle a large skillet with olive oil. Cook the onion over medium-high heat until translucent about 3 minutes.
Add the ground beef and sausage to the pan. Add 1 tablespoon Italian seasoning, garlic salt and red pepper flakes.
Cook over medium-high until no pink remains. Drain any excess fat from the pan.
Add the marinara sauce and tomato paste. Cook stirring just until the tomato paste has dissolved. Remove from the heat. (This can be done in advance and chilled until assembling)
In a small mixing bowl mix together the ricotta, beaten egg, grated Parmesan and remaining 1 teaspoon Italian seasoning.
To assemble: Spray the crock of a 6-quart oval slow cooker with cooking spray. Spread 1 cup meat sauce sauce on the bottom from edge to edge. Arrange 3 lasagna noodles in a single layer over the sauce. Break to fit as needed.
Spread noodles with ⅓ of the ricotta, ⅓ of the meat sauce, 1 cup shredded cheese.
Arrange 10 pepperoni slices over the cheese. Repeat 2 more times.
End with the 4th layer of noodles and cover liberally with sauce.
Cover and cook on low for 4-6 hours. (See Cook's note)
Uncover and top with remaining 1 cup cheese. Cover and let rest just until cheese melts. Let stand uncovered for 10 minutes.
Garnish with fresh basil, if desired. Cut and serve.