In a medium bowl, combine ricotta cheese, minced garlic, dried oregano, black pepper, red pepper flakes (if using), and shredded Parmesan cheese. Mix well and set aside.

In another medium bowl, mix drained diced tomatoes, pasta sauce, and chopped fresh parsley. Set aside.

Spread about 1 cup of the tomato mixture onto the bottom of a 6-quart slow cooker to just cover the bottom.

Add a layer of lasagna noodles on top, breaking them as needed to fit the slow cooker.

Add half of the baby spinach over the noodles.

Dollop 1/3 of the ricotta mixture over the spinach, loosely spreading it with a spatula.

Top with 1 cup of shredded mozzarella cheese, followed by 1/3 of the remaining tomato mixture.

Add another layer of noodles, then repeat the layering process with the remaining spinach, ricotta mixture, mozzarella cheese, and tomato mixture.

Finish with a final noodle layer, followed by the remaining ricotta, tomatoes, and the last of the mozzarella cheese.

Cover the slow cooker and cook on LOW for 3 hours, or until the noodles are tender.

Remove the lid and let the lasagna rest for at least 15 minutes before slicing and serving.