Dice the onion and bell pepper. Sautee the veggies in 1 tbsp of butter or oil until softened.
Add the sliced beef and cook through, removing any accumulated liquid.
With the heat still on low, add the cream cheese and mix through until combined and saucy.
Add the garlic powder and season with salt and pepper to taste. Set aside to cool.
Prepare the mushroom caps by washing the mushrooms and then pop out the stems.
Put a generous amount of filling in each mushroom cap - how much will depend on the size of the mushroom. Fill them so that there's a bit of mounded filling at the top. Place the filled mushrooms on a sheet pan.
Top the mushroom caps with the shredded provolone cheese.
Bake in a preheated 350-F oven for about 20 minutes or until hot and bubbly.