Remove stem and gills from mushrooms, using a spoon to gently scrape the out. Drizzle with olive oil and season with salt and pepper, set aside.
Sauté the onion, garlic, green pepper, celery, and chili powder in 2 Tbsp of oil until onion is softened. Remove and set aside.
Fry the ground beef until fully cooked, drain any fat. Add in remaining ingredients, and simmer until mixture has thickened, about 15 minutes. Season to taste with salt and pepper.
While meat mixture is cooking, prepare the mushroom caps. Grill them on each side using; the stovetop (as pictured), in the oven under the broiler, or you can use a panini press. Grill mushrooms until they are slightly softened, then set aside.
To serve, on top of each mushroom (gill-side up) add 3/4 cup of Sloppy Jop mixture.
Notes
-To make this vegetarian, replace meat with 1-2 cups of cooked red quinoa. -If can't find GF liquid bouillon, use powdered version mixed with a bit of water.