Preheat and Prepare: First, preheat your oven to 180°C (160°C Fan / Gas Mark 4). Lightly grease a 23x33cm (9x13 inch) baking dish with a little oil or butter.

Sauté the Vegetables: Heat the olive oil in a large frying pan over a medium heat. Add the chopped onion, red pepper, and courgette. Cook for 5-7 minutes, stirring occasionally, until the vegetables have softened.

Wilt the Spinach: Add the fresh spinach to the pan in handfuls. It will look like a lot, but it wilts down quickly! Stir for 1-2 minutes until it has completely wilted. Drain away any excess liquid that has been released.

Assemble the Base: Spread the cooked vegetable mixture evenly across the bottom of your prepared baking dish. Scatter the sliced or crumbled turkey sausage over the vegetables.

Create the Egg Mixture: In a large bowl, whisk together the 12 eggs until the yolks and whites are well combined. Add the cottage cheese, milk, Dijon mustard, smoked paprika, salt, and pepper. Whisk again until everything is smooth and incorporated. A few small lumps of cottage cheese are perfectly fine.

Combine and Bake: Carefully pour the egg mixture over the vegetables and sausage in the baking dish. Gently tilt the dish to ensure the mixture is evenly distributed.

Add the Cheese: Sprinkle the grated mature cheddar evenly over the top.

Bake to Perfection: Place the dish in the preheated oven and bake for 35-40 minutes. The casserole is ready when the centre is set (a knife inserted into the middle should come out clean) and the top is golden brown.

Rest Before Serving: Remove the casserole from the oven and let it rest for at least 10 minutes before slicing. This allows it to set properly, making it much easier to cut into clean squares.