This is a very versatile dish. You can:
Use any grain you wish. Just follow the directions on the package.
Use any bean that strikes your legume fancy.
Use any combination of veggies for the mixture. Celery, carrot, leek, peas are good options.
Use any color of pepper.
Switch up the herbs and add a little spice like cayenne or red pepper.
Sprinkle the filled peppers with chopped almonds, for a little extra crunch.
When almost cooked through, remove cover, let sit for ten minutes and then top with some plant based cheese and return to the oven for ten more minutes.
Make ahead. Bake for 20 minutes and then cool and refrigerate. Return to room temp. and reheat for 20-30 minutes in a 325 degree oven.