Slice the flank steak into thin, bite-sized strips against the grain.
Heat the olive oil in a large skillet over medium-high heat.
Add the steak strips, cumin, garlic powder, sea salt, and black pepper to the skillet, sautéing until browned for 5-7 minutes.
Add the sliced bell peppers and red onion to the skillet, cooking for 5 minutes until tender-crisp.
Stir in the rinsed black beans and cooked brown rice, heating for 3-4 minutes until the mixture is well-combined and hot.
Warm the whole wheat tortillas in a separate dry pan or microwave for 15 seconds to make them pliable.
Divide the steak, bean, and rice mixture evenly among the four tortillas.
Top each burrito with shredded cheddar cheese, fresh salsa, a dollop of Greek yogurt, and sliced avocado.
Fold the sides of each tortilla inward and roll tightly to secure the filling.
Serve two burritos immediately and store the remaining two in an airtight container for leftovers.