Gruyere and Sausage Omelette

Butter, for greasing baking dish

2 Tbsps olive oil

1 small onion, diced

8oz to 1lb pound mild Italian sausage, casings removed

8 large eggs

½ cup whole milk

½ to ¾ tsp salt

Freshly ground black pepper

1 + ½ cup (4oz) grated Gruyere cheese

1⁄4 cup chopped fresh parsley or lovage (optional: + 2 Tbsps to sprinkle on top) 

1. Preheat the oven to 425°F. Lightly butter an 8 by 8" baking dish.
2. Heat the oil in a medium skillet over medium-high heat. Add the onion and cook until soft. Add the sausage and cook until starting to brown. Set aside to cool.
3. In a large bowl, whisk together the eggs, milk, salt and pepper. Add 1 cup of Gruyere cheese and ¼ cup of parsley. Stir in the onion mixture. Pour the batter into the prepared pan. Sprinkle with the remaining cheese. Bake for 20 to 25 minutes or until golden brown.
4. Sprinkle with remaining parsley before serving.