In a shallow dish, whisk together buttermilk, egg, and Cajun seasoning. Place flour in another shallow dish. Dip fish in buttermilk and flour. Repeat to coat fish twice.
Heat 3 tablespoons of the oil in a large heavy skillet over medium-high heat. Carefully add fish to hot oil (working in batches, if necessary). Cook for 3 to 5 minutes on each side or until golden. Add more oil, if needed. Drain on paper towels.
Drain oil from skillet; wipe clean with paper towel. Add peppers to skillet and cook 2 minutes or until crisp tender.
Serve fish with peppers and lemon. Makes 4 servings.