Bring large cast-iron skillet to medium high heat. Add oil, swirl to coat the pan.
Add garlic and move around pan until it becomes fragrant, approx. 1 minute.
Add onion, bell peppers and the fajita seasoning. Stir to combine. Add additional olive oil to pan if you feel it is getting a bit dry.
Sauté for approx. 10-15 minutes or until the peppers have reached your desired doneness (cook shorter for a more firm, but tender pepper - longer for a softer pepper).
Taste and add any additional salt or pepper, to taste.