Preheat the oven to 375°F or 190°C and grease a 9×13-inch baking dish. Pat the thawed hash browns dry before using them.
Cook the onion and bell pepper in a lightly oiled skillet for 4 to 5 minutes. Remove them from the heat once they begin to soften.
Whisk the eggs, milk, Dijon mustard, garlic powder, salt, and black pepper. Add the ham, vegetables, hash browns, cheese, and chives.
Pour the mixture into the baking dish and spread the ingredients evenly. Make sure the ham and potatoes are distributed throughout.
Bake for 40 to 45 minutes, until the center no longer jiggles and the top is lightly browned. Avoid overbaking so the eggs remain tender.
Let the casserole rest for 10 minutes before slicing. Cool completely before placing individual portions in freezer-safe containers.