Soak the cedar plank in water for 30 minutes.
Fire up your grill for direct and indirect cooking.
Meanwhile, lightly brush the mushrooms on both sides with oil.
Place on grill, gill-side up, over direct heat for 3 minutes. Remove and let cool slightly while the plank continues to soak.
Transfer portobellos to the plank.
Spoon pork chorizo stuffing into mushroom tops.
Place on grill over indirect heat and cook for 15 minutes. Check often to make sure the plank doesn't catch fire. If it does, just blow on it or spritz it with a little water to extinguish.
Add the cheese and cook another 5 minutes or until melted.
Sprinkle with chopped cilantro and serve.