Preheat oven to 375˚F. Lightly grease a 9”x13" baking dish and set aside.
Place a skillet over medium heat. Add ½ tablespoon oil and brown sausage, breaking up as it cooks, 5 to 7 minutes. Using a slotted spoon, transfer sausage to a large mixing bowl. Discard and excess fat and liquid from pan.
Add remaining oil to pan and sauté onions and bell pepper for 5 to 6 minutes, until vegetables just begin to caramelize. Season with salt and pepper and stir together. Transfer vegetables to bowl with sausage.
Add diced potatoes, ⅔ of shredded cheese, 2 tablespoons chives, and salt and pepper to sausage and vegetable mixture and gently toss together until fully combined.
In another bowl whisk together eggs and milk and pour mixture over sausage vegetable mixture. Fold together until just combined.
Pour mixture into prepared baking dish and sprinkle top with remaining cheese.
Cover casserole tightly with foil and bake for 35 minutes. Remove foil and continue to bake for an additional 10 minutes or until eggs are set.
Top casserole with remaining chives, black pepper and serve.
