Brown your ground beef and breakfast sausage in a 12" cast iron skillet over medium heat, then drain and return to pan.
Add diced onion and minced garlic and saute another 5-7 minutes over medium heat or until onion is soft, stirring occasionally.
While meat is browning, clean mushroom caps, remove stems, and pat dry.
Place cup side up on a rimmed baking pan lined with foil or silicon baking mats and brush with olive oil, then sprinkle generously with oregano and Italian seasoning.
Bake at 400 for 20 minutes, then drain off excess liquid and set aside.
Stir tomatoes, oregano, basil, crushed red pepper, 1 Tbsp parsley, and 1/2 tsp salt into browned beef & sausage mixture, then simmer on low for 10-15 minutes.
While your meat sauce is simmering, combine ricotta, 1/2 Tbsp parsley, Parmesan, sea salt, & pepper in separate bowl.
Arrange portobello caps cup side up on top of your finished meat sauce.
Fill each with ricotta-Parmesan mixture, then top with sliced mozzarella.
Bake at 400 degrees for 35 minutes or until cheese is nicely browned.
Garnish with chopped fresh parsley (optional).
If you freeze your mozzarella for 20 minutes or so, it will slice more easily. Use caution when removing your finished portobello skillet lasagna from the oven, since it will be very full, very hot, and will spill over the edges very easily. Lastly, don’t skip the step of precooking the mushroom caps, since you don’t need all that excess liquid in your finished dish.
Low Carb Portobello Skillet Lasagna from Mashup Mom: https://www.mashupmom.com/low-carb-portobello-skillet-lasagna/