Heat a large skillet over medium-high heat. Add the ground beef and diced onion and cook, breaking up the meat, for 7-8 minutes until the beef is browned and the onion is softened. Drain excess fat.

Add the minced garlic, Italian seasoning, salt, and pepper. Cook for 1 minute until fragrant. Remove from heat and stir in the marinara sauce. Set aside.

In a medium bowl, combine the ricotta cheese, egg, 1/4 cup of the Parmesan cheese, Italian seasoning, garlic powder, and salt. Stir until smooth and well combined. Set aside.

Lightly spray the inside of a 6-quart slow cooker with nonstick cooking spray, or line it with a sheet of parchment paper with overhang on the sides for easy removal and cleanup.

Spread about 1/2 cup of the meat sauce in an even layer on the bottom of the slow cooker to prevent sticking.

Break no-boil lasagna noodles to fit and arrange them in a single layer over the sauce, covering as much of the surface as possible. It's okay to overlap slightly.

Spread half of the ricotta mixture over the noodles, then sprinkle with 1/2 cup of the shredded mozzarella. Add another layer of meat sauce on top.

Repeat the layering: noodles, remaining ricotta mixture, 1/2 cup mozzarella, meat sauce.

Add one final layer of noodles on top, then pour the remaining meat sauce over everything, making sure the noodles are fully covered and coated with sauce. Top with the remaining 1 cup mozzarella and remaining 1/4 cup Parmesan.

Place the lid on the slow cooker and cook on LOW for 3.5 to 4.5 hours, or until the noodles are completely tender when pierced with a fork and the cheese is melted. Do not cook on HIGH — LOW heat gives the best texture.

Turn the slow cooker off and remove the lid. Let the lasagna rest uncovered for 15-20 minutes before slicing — this is essential for the layers to firm up and hold their shape when served.

Garnish with fresh basil or parsley if desired. Slice and serve directly from the slow cooker, or lift out using the parchment overhang for clean slices on a cutting board.