Heat a cast iron skillet over medium-high heat until a sprinkle of water instantly evaporates when it hits the pan. Add a generous TBSP of canola oil to the skillet and immediately add the onions and bell peppers. Stir-fry, turning frequently and keeping the heat high for approximately 3 minutes, or until the peppers and onions begin to soften and char. Remove the peppers and onions and put them on a serving platter. Add the remaining canola oil and then the beef, stir-frying quickly until the beef is nearly cooked through, approximately 3 minutes. Return the bell peppers and onions to the skillet and stir-fry the peppers, onions and beef together for 1 minute. Remove from the heat, and spoon the beef, peppers and onions into warm corn tortillas.