Prepare the Base: First, generously butter a 9x13 inch (or similar large) baking dish. Spread the bread cubes in an even layer across the bottom of the dish. Set it aside for now.

Cook the Sausage and Vegetables: Place a large frying pan over a medium-high heat. Add the sausage meat (casings removed) and cook, breaking it up with a wooden spoon, until it's nicely browned. This should take about 5-7 minutes. Add the chopped onion and bell pepper to the pan and continue to cook for another 5 minutes, until they begin to soften.

Sauté the Aromatics: Add the sliced mushrooms and minced garlic to the pan. Cook for a further 4-5 minutes, until the mushrooms have released their liquid and started to brown. Season the mixture with a pinch of salt and pepper. Drain off any excess fat from the pan.

Layer the Casserole: Spoon the cooked sausage and vegetable mixture evenly over the bread cubes in your prepared baking dish. Sprinkle about two-thirds of the grated Cheddar cheese over the top.

Create the Egg Custard: In a large mixing bowl, whisk together the 10 eggs until they are well beaten. Whisk in the whole milk, double cream, Dijon mustard, chopped parsley, 1 tsp of salt, and ½ tsp of black pepper. Continue whisking until everything is thoroughly combined and the mixture is smooth.

Assemble and Rest: Carefully pour the egg custard mixture evenly over the layers in the baking dish, making sure to cover all the bread. Gently press down on the bread with a spatula to help it soak up the liquid. At this point, you can either proceed to baking or cover the dish and refrigerate it overnight. Letting it rest is highly recommended!

Bake to Perfection: When ready to bake, preheat your oven to 180°C (160°C Fan / Gas Mark 4). Sprinkle the remaining cheese over the top of the casserole. Bake, uncovered, for 45-55 minutes. The breakfast casserole is ready when the centre is set (a knife inserted should come out clean) and the top is golden brown and puffy.

Rest and Serve: Let the casserole rest for at least 10 minutes before slicing and serving. This allows it to set properly, making it much easier to cut into clean portions. Garnish with a little extra fresh parsley if you like.