In a small bowl, mix together the seasoning ingredients. You can mix them in two separate bowls - half will season the veggies, and the second half will season the meat.
Heat the oil in a large, deep skillet over medium-high heat. Add the bell peppers, onions, and half the spices. Cook, stirring often, until just tender, about 5 minutes. Remove the vegetables to a plate.
Add the steak slices and the remaining spices to the same skillet. Stir-fry just until no longer raw, 3-4 minutes. The beef doesn't need to be fully cooked at this point. It will finish cooking when you add back the vegetables.
Return the vegetables to the skillet. Stir-fry everything together until heated through, 1-2 more minutes.At this point, if you have a lot of liquids in the skillet (this happens when using frozen peppers) and you'd like to thicken them, you can reduce the heat to medium and stir in a cornstarch slurry (1 tablespoon of cornstarch mixed with 1 ½ tablespoons of cold water). Keep cooking until the liquids thicken, about 1 more minute. But this is optional and unnecessary if using fresh peppers.
Serve immediately.
