In a skillet, over medium heat, add the oil, onion, garlic, and mushroom stems. Cook for 2 - 3 minutes, until onion begins to soften.
Add the sausage patties and crumble with a wooden spoon. Season with salt, pepper, and paprika, and cook until lightly browned through.
Transfer the sausage to a heavy mixing bowl along with the parsley and Gruyere cheese. Stir to combine ingredients and stuff 1 TBS of mixture into each mushroom cap. Place into the crock pot, cover and cook on High for 2 hours Serves 6 - 10.