Yes these beef stuffed portobellos can be made as an appetizer! You can use crimini mushrooms (which are actually immature portobellos) to make this an appetizer recipe. Carefully remove the stems with a pairing knife and then scrape out any gills to make space for the filling. Make sure that the mushrooms don't topple over in the skillet by slicing a small sliver off the rounded edge of the cap so they stand up and don't roll. Bake them until an instant read thermometer registers 165 degrees F in the center of the filling. Also check out our Healthy Stuffed Mushrooms appetizer recipe as well.