Preheat oven to 325 degrees F. You will need either a 13x9 or 11x7-inch baking pan (or any that will fit the peppers and the remaining quinoa mixture)
Cook quinoa according to package directions using 1 cup uncooked quinoa in 1 1/2 cups water. Quinoa cooks like rice. I used my rice cooker for this.
Cut off and reserve the tops of the bell peppers. Hollow them out then steam both the tops and the peppers in the remaining 1/2 cup of water until slightly soft but not limp; drain and cool slightly.
In a large skillet or saute pan, melt the butter over medium heat. Add the mushrooms and onion and saute until tender.
Drain the can of diced tomatoes, but reserve the juice. Add the drained tomatoes, garlic and the salsa to the onion/mushroom mixture. Stir well and simmer uncovered for 10 minutes.
Add the sherry and stir well. Fold in the cooked quinoa and the mozzarella cheese. Stuff peppers with the quinoa mixture--you will use about half of the mixture to stuff the peppers. Place stuffed peppers upright in a baking dish and sprinkle tops of each pepper with reserved cheese.
Stir the reserved juice from the can of tomatoes into the remaining half of the quinoa mixture and pour around peppers in the baking pan.
Bake uncovered for 20-25 minutes. Top with reserved pepper tops and bake an additional 10-15 minutes.
Serves 4-6 depending on size of the peppers.