Preheat oven to 400º. Line a baking sheet with aluminum foil or grease a baking sheet (or both!) and set aside.
Wash, dry and remove stems from Portabella caps. Use a pairing knife to cut back flesh that covers gills, if needed.
In a large skillet, add oil and once hot add in all chopped vegetables and spices.
Cook for 4-5 minutes until the veggies are tender and then remove from the heat.
Place cooked veggies in a large bowl and add prepared rice. Mix well.
Add ground beef (uncooked) to rice and veggies and mix until well incorporated.
Place Portabella caps on prepared baking sheet and evenly divide the mixture into balls and place into mushroom caps, being careful to spread out the mixture to edge of cap.
Spray or drizzle edges of mushroom caps with oil and cook for 25 minutes or until thermometer reads 160º.
Enjoy with avocado, pico de gallo, cilantro or other favorite toppings!