In a skillet, heat the olive oil over medium heat. Add the onion, mushrooms, and tomato paste and saute until the mushrooms are golden brown. Add the cauliflower rice, sea salt, and black pepper and stir. Cook for 5 minutes.

Fill the halves of each bell pepper with the cauliflower mixture.

In a 4- to 6-quart slow cooker, arrange the stuffed peppers in a single layer. Cover and cook on LOW heat for 4 hours.

Serve with Parmesan cheese.