Preheat Oven and Prepare Pie Plate: Begin by preheating your oven to 375 degrees F. Coat a 9-inch deep-dish pie plate with non-stick cooking spray to ensure easy removal of the finished dish.
Sauté Ingredients: Add vegetable oil to a 12-inch skillet and place it over medium heat. Add diced onion, and sauté, stirring occasionally, for about 3 minutes or until the onions are softened. Stir in the ham and garlic with the onions, cooking for an additional minute or two until the ham is warmed through and the garlic is fragrant. Transfer this mixture into the prepared pie dish, spreading it out evenly across the bottom.
Prepare the Egg Mixture: In a large mixing bowl, whisk together the eggs, milk (or cream), ½ teaspoon of salt, ½ teaspoon of pepper, and flour until well combined. This will form the custard base of your dish.
Assemble the Dish: Pour the egg mixture over the ham and onions in the pie plate, ensuring it evenly covers the ingredients. Top with a generous sprinkle of Gruyere, cheddar cheese, and chives for a flavorful finish.
Bake and Serve: Bake in the preheated oven for 30 to 40 minutes, or until the top is golden brown and the center is fully set. Once baked, allow the dish to cool slightly before cutting it into wedges. Serve warm for the best flavor and enjoyment.
