1) Prepare your grill for indirect heat grilling. If you’re using charcoal, heat enough coals for one hour of cooking. Push the coals to one side of the grill.
If you’re using gas, turn on one burner on medium-low (either the far side or the back burner of the grill depending on your model).
2) Scrub mushrooms well. Use a paring knife to hollow out the center of each mushroom. Be careful not to cut through the mushrooms. Drizzle mushrooms well with olive oil on all sides.
3) In a medium bowl, stir together red onions, chopped artichoke hearts, sun-dried tomatoes, and grated cheese. Season mixture with a pinch of salt and pepper.
4) When ready to grill, place mushrooms (unstuffed) stem-side down over direct heat. Grill for 1 minute until mushrooms have nice grill marks.
5) Flip mushrooms and transfer to an indirect heat area of the grill. Spoon filling into each mushroom. Go heavy on the filling. It’s okay if you lose some, but you should use it all.
6) Sprinkle each mushroom with breadcrumbs and cook for 20 minutes, covered, over indirect heat. Rotate the mushrooms once halfway through to make sure they are cooking evenly.
Mushrooms are done when the mushroom is tender and the cheese is melted in the filling. Serve mushrooms while warm.
OPTIONAL: If you don’t feel like firing up the grill, you can assemble the mushrooms and bake them at 350 degrees F. for 25-30 minutes.