Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper for easier cleanup, if desired. Prepare the portobello mushroom caps by gently scraping out the dark gills with a spoon. Brush the outsides and insides of the mushroom caps with a little olive oil and season lightly with salt and pepper. Place them hollow-side up on the prepared baking sheet.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the chopped onion and bell pepper and sauté for 3-5 minutes until softened. Add the minced garlic and cook for another minute until fragrant. Push the vegetables to one side of the skillet.
Add the ground Italian sausage (or other ground meat) to the other side of the skillet. Break it up with a spoon and cook until browned, about 5-7 minutes. Once cooked, drain any excess grease from the skillet. Stir the cooked meat and vegetables together.
Remove the skillet from the heat. Stir in the sugar-free marinara sauce, 1 cup of the shredded mozzarella cheese, grated Parmesan cheese, Italian seasoning, salt, and black pepper into the meat and vegetable mixture. Mix well to combine all ingredients thoroughly.
Evenly spoon the pizza filling mixture into the hollows of the prepared portobello mushroom caps. Top each stuffed mushroom with the remaining 1/2 cup of shredded mozzarella cheese. If using, arrange pepperoni slices or chopped pepperoni on top of the cheese.
Bake for 20-25 minutes, or until the mushrooms are tender and the cheese is melted, bubbly, and lightly golden. If you prefer a more browned top, you can broil for the last 1-2 minutes, watching carefully to prevent burning.
Carefully remove the baking sheet from the oven. Let the stuffed mushrooms cool for a few minutes before serving. Garnish with fresh chopped basil, if desired. Serve hot and enjoy your keto pizza!
Notes
For a vegetarian option, omit the ground sausage and replace with a mix of sautéed zucchini, eggplant, or extra bell peppers. You can also add other keto-friendly toppings like sliced black olives, jalapeños, or crumbled cooked bacon. Leftovers can be stored in an airtight container in the refrigerator for up to 3 days and reheated gently in the oven or microwave.