Preheat your oven to 375 degrees f.
Cook the brown rice according to package instructions.
Cut the tops off of the bell peppers and using a paring knife carefully remove and discard the seeds and pit.
Cut out a heart in the center of each bell pepper as shown in the photos; save the cutout hearts.
Place your peppers into a baking dish or tray and lightly spray with cooking oil. Sprinkle with sea salt and pepper to taste. Bake at 375 degrees f. for 10-12 minutes, or just until peppers slightly soften.
Meanwhile, in a large skillet heat your oil over medium-high heat. Add in onion, garlic, celery, carrots and sauté for 3-4 minutes. Stir in the yellow bell pepper and mushrooms and continue to cook for 3-4 minutes, stirring occasionally.
Turn your heat off, and add the cooked rice to the skillet together with the remaining ingredients.
Once the time is up remove your red peppers from the oven and stuff each with the rice mixture.
Fit the cutout hearts back into the peppers to prevent the stuffing from spilling out.
Return to the oven and bake for 18-20 minutes more, or until cheese is melted and everything is hot throughout.
To serve, remove the cutout hearts to expose the filling of each pepper.
Serve warm and enjoy!