Preheat the oven to 300°F (149°C). Grease a 9-x-13-in metal baking dish with 1 tbsp of the butter.
Heat remaining butter in a medium frying pan over medium-high heat. Add onion and peppers and cook 3 to 5 minutes or until slightly softened and tinged with gold.
Whisk eggs with salt in a large bowl. Season with pepper. Add onion mixture and ham and stir until well combined. Pour into prepared pan then use a spoon to distribute the ingredients evenly across the pan. Bake 17 to 22 minutes or until just set. Let stand 5 minutes then cut into 6 squares and cool completely.
Loosen from the pan with an offset or thin metal spatula. Stack, separated by squares of parchment paper, in a resealable container. Keep refrigerated up to 3 days.
Reheat gently in a non-stick frying pan or in short intervals in the microwave.
Nutrients per serving: 208 calories, 11 g protein, 16 g fat (7 g saturated fat), 4 g carbohydrates, 438 mg cholesterol, 398 mg sodium
Best Health tip: Sandwich between pieces of toast with a slice of cheese if desired.