Heat ½ tablespoon of the canola oil in a large skillet over medium-high heat.
Add the sliced onion and one of the bell peppers to the pan. (We're cooking in batches so we get a nice char on the veggies instead of overcrowding the pan and just steaming them.) Sprinkle with ½ tablespoon of fajita seasoning. Cook, stirring occasionally, until the veggies are tender and lightly charred, about 6-7 minutes. Remove to a bowl.
Heat the remaining ½ tablespoon of canola oil in the same pan. Add the other sliced bell pepper and the zucchini to the pan, along with ½ tablespoon of fajita seasoning. Cook, stirring occasionally, until the veggies are tender and lightly charred, about 6-7 minutes.
Add the onion and pepper back to the pan and add the black beans, diced green chilies and remaining tablespoon of fajita seasoning. Stir to mix well. Cook 1-2 minutes, until warmed through.
Serve immediately with tortilla shells and desired toppings.
