Prepare and cook brown rice according to package directions.
While rice is cooking: In a large skillet over medium heat, cook ground beef and diced onion 5-7 minutes until browned/no pink left and onions are soft, stirring to break ground beef into small pieces. (Measure spices during this time to decrease prep time) Drain excess grease.
Add pinto beans, diced tomatoes, tomato sauce, worcestershire sauce, vinegar, and all spices (brown sugar, garlic & onion powder, salt & pepper). Turn to low heat and simmer for 10 minutes, stirring occasionally, and prepare the bell peppers.
Prepare bell peppers by slicing in half once down the middle lengthwise and removing seeds and stem to make a boat shape. To decrease cooking time in the oven, steam in the microwave by placing upright lengthwise in a microwave safe container, adding ½ cup of water to the bottom, covering and cooking for 4 minutes. The peppers should be semi-soft (not crunchy but not mushy).
Assemble the stuffed peppers:
Preheat the oven to 350 F.
Place peppers open side up in a 9x13 baking dish. Layer the bottom of the peppers with 2 Tb shredded mozzarella cheese (about 1 cup between 8 peppers).
Scoop ¼ cup rice into the bottom of each pepper, then ½ cup ground beef mixture into each pepper. Last, layer the remaining cheese evenly over the peppers, adding more as desired.
Bake at 350 F. for 15-20 minutes until cheese is melted, filling is heated through and peppers are soft. Optional: sprinkle chopped green onion on tops of peppers to serve.
Notes
Refrigerate: for up to 5 days covered. Freeze: for up to 3 months in a freezer-safe container. You can use white rice, quinoa, or minute rice. Just make sure to cook according to package directions, as water amount and cooking times can be different.