Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Lightly grease a 23x33cm (9x13 inch) baking dish with butter or a little oil.

Cook the Sausage and Onion: Heat the olive oil in a large frying pan over a medium heat. Add the chopped onion and cook for 4-5 minutes until it begins to soften. Add the sausage meat to the pan, breaking it up with a wooden spoon. Cook for 8-10 minutes, until it's browned all over and no pink remains. Drain off any excess fat and set aside.

Create the Egg Base: In a large mixing bowl, crack the 10 eggs and whisk them lightly, just enough to break up the yolks.

Blend the Cottage Cheese: Place the cottage cheese in a blender or food processor. Blend for 30-60 seconds until it is completely smooth and creamy, with no lumps remaining. This step is essential for achieving the best texture. Scrape the smooth cottage cheese into the bowl with the eggs.

Combine the Wet Ingredients: To the egg and cottage cheese mixture, add the whole milk, Dijon mustard, salt, pepper, and freshly grated nutmeg. Whisk everything together until it's well combined and uniform in colour.

Add the Fillings: Stir the cooked sausage and onion mixture, the sliced spring onions, and about two-thirds of both the cheddar and Gruyère cheeses into the egg mixture.

Assemble the Casserole: Pour the entire mixture into your prepared baking dish and spread it out evenly. Sprinkle the remaining grated cheese over the top. What works best for me is ensuring the cheese covers the surface right to the edges for a lovely, crispy crust.

Bake Until Golden: Place the dish in the preheated oven and bake for 35-40 minutes. The bake is ready when the centre is just set (it shouldn't jiggle when gently shaken) and the top is puffed and golden brown.

Rest Before Serving: Remove the egg bake from the oven and let it rest on a wire rack for at least 10 minutes before slicing and serving. This allows it to set up properly, making it much easier to cut into clean squares.