Prepare Your Pan & Oven: Lightly grease a 23x33cm (9x13 inch) baking dish. If you're baking immediately, pre-heat your oven to 180°C (160°C fan/Gas Mark 4).

Cook the Sausage: Heat the olive oil in a large frying pan or skillet over a medium-high heat. Add the sausage meat, breaking it up with a spoon, and cook until it's browned and cooked through, about 5-7 minutes. Drain off any excess fat and set the sausage aside.

Sauté the Vegetables: In the same pan, add the chopped onion and red bell pepper. Sauté for 5-7 minutes until softened. Add the sliced mushrooms and cook for another 3-4 minutes until they release their liquid and begin to brown. Finally, stir in the minced garlic and cook for just 1 minute more until fragrant. Remove from the heat.

Whisk the Egg Mixture: In a large mixing bowl, whisk together the eggs, milk, mixed herbs, salt, and pepper until well combined. Stir in the grated cheddar and mozzarella cheeses, then add the chopped baby spinach. If you fancy a little heat, this is the time to add a pinch of chilli flakes.

Assemble the Casserole: Evenly spread the cooked sausage and sautéed vegetables across the bottom of your prepared baking dish. Pour the egg and cheese mixture over the top, ensuring everything is well distributed and submerged.

Bake or Chill: To Bake Immediately: Place the dish in the pre-heated oven and bake for 45-50 minutes, or until the centre is set and the top is golden brown. A knife inserted into the centre should come out clean.

To Make Ahead: Cover the baking dish tightly with foil or cling film and refrigerate for up to 24 hours. When ready to bake, remove from the fridge 20-30 minutes before baking to allow it to come closer to room temperature. Bake as above, but you might need an extra 5-10 minutes of cooking time.

Rest and Serve: Once baked, remove the casserole from the oven and let it rest for 5-10 minutes before slicing and serving. This resting period allows the casserole to set further, making for cleaner slices. Garnish with fresh chopped parsley.