Mix ground beef with salt and pepper and desired spices. Sometimes I add some italian spices, or cumin, or chili powder, depending on my mood. Make into 4 patties.
Clean/rinse mushrooms, dry, and gently rub with olive oil.
Place mushrooms and burgers on grill for 10-15 minutes until desired doneness.
Serve with toppings - lettuce/spinach, tomatoes, etc. (use whole30 approved mayo, ketchup, and mustard if desired)
Recipe by One Green Fork at https://www.onegreenfork.com/hamburger-portabello-bun/