Prepare Your Dish & Preheat Oven: Lightly grease a 9x13 inch (approximately 23x33 cm) baking dish with butter or cooking spray. Preheat your oven to 180°C (160°C fan/Gas Mark 4). This ensures the oven is at the correct temperature for even cooking from the moment your casserole goes in.

Sauté the Vegetables: Heat the olive oil in a large frying pan or skillet over a medium heat. Add the chopped onion and cook for about 5-7 minutes until softened and translucent. Stir in the minced garlic and bell peppers, cooking for another 5 minutes until they begin to soften. Add the sliced mushrooms and cook until they release their moisture and start to brown, about 5-7 minutes. Finally, stir in the fresh spinach and cook just until it wilts, which should only take a couple of minutes. Remove from the heat. Sautéing the vegetables beforehand is crucial; it removes excess moisture that could make your casserole watery and develops their flavour.

Whisk the Egg Mixture: In a large mixing bowl, crack the 8 large eggs. Add the semi-skimmed milk, dried mixed herbs (or fresh parsley), salt, and black pepper. Whisk vigorously until the eggs are thoroughly combined and the mixture is light and frothy. Proper whisking ensures a light and airy texture in the final casserole.

Assemble the Casserole: Scatter the cubed stale wholemeal bread evenly over the bottom of your prepared baking dish. Next, layer the sautéed vegetable mixture over the bread. If using, sprinkle the cooked lean turkey sausage, bacon, or vegetarian alternative evenly over the vegetables. Sprinkle about two-thirds of the grated mature Cheddar cheese over the layers. Finally, pour the whisked egg and milk mixture carefully and evenly over everything in the dish. Gently press down on the bread and vegetables with the back of a spoon to ensure they are well submerged in the egg mixture. This helps the bread soak up all that eggy goodness.

Chill (Optional, but Recommended for Make-Ahead): For best results and for a truly make-ahead healthy breakfast casserole, cover the baking dish tightly with aluminium foil and refrigerate for at least 30 minutes, or preferably overnight (up to 24 hours). This allows the bread to fully absorb the liquid, leading to a more cohesive and evenly baked casserole. If baking immediately, proceed to the next step.

Bake the Casserole: If baking from chilled, remove the casserole from the fridge about 20-30 minutes before baking to let it come closer to room temperature. Cover the baking dish loosely with aluminium foil and bake for 30 minutes. This initial covered bake helps the casserole cook through without the top browning too quickly. After 30 minutes, remove the foil, sprinkle the remaining one-third of the grated cheese over the top, and continue to bake uncovered for another 15-20 minutes, or until the casserole is golden brown on top, puffed up, and a knife inserted into the centre comes out clean. The internal temperature should reach 71°C (160°F) for safety.

Rest and Serve: Once baked, remove the healthy breakfast casserole from the oven and let it rest for 5-10 minutes before slicing and serving. This resting period is crucial; it allows the eggs to set completely, making for cleaner slices and a less crumbly texture. My husband, who's usually picky about breakfast, actually asked for seconds the first time I made this with a good rest! Garnish with a sprinkle of fresh chopped parsley or chives, if desired.