I found this recipe on Wash You Dry and adapted to a vegetarian version.
Broccoli Cheese Stuffed Portobellos
A quick and easy vegetarian dinner.
Wash and take out the membrane using a spoon.
Season with the salt, pepper, garlic powder, and paprika.
Put the broccoli in bowl add a little bit of water and cover with saran wrap. Microwave for 2 minutes.
Mix the red pepper, cheddar cheese, mayo and broccoli. Once mixed stuff the portobellos.
Cook the portobellos at 350 until cooked, should take about 15 minutes. You can either cook it on baking pan or casserole dish.