1 Preheat the oven to 180°C (350°F). Grease a 9×13-inch (23×33 cm) baking dish with butter or cooking spray.

2 Cook the bacon in a skillet over medium heat until crisp. Transfer to a paper towel-lined plate to drain, then chop into pieces. Reserve 1 tablespoon of bacon fat and discard the rest or save for another use.

3 In the same skillet, add the butter and reserved bacon fat if desired. Sauté the chopped onion over medium heat until translucent, about 4–5 minutes.

4 Add the fresh spinach to the skillet and cook, stirring, until just wilted, about 2–3 minutes. Season lightly with salt and pepper. Remove from heat and let cool slightly.

5 In a large bowl, whisk together the eggs, milk, Dijon mustard, nutmeg, salt and pepper until well combined.

6 Spread half of the cubed bread evenly in the prepared baking dish. Sprinkle half of the grated Gruyère over the bread, then add the wilted spinach and chopped bacon. Top with the remaining bread and cheese.

7 Pour the egg and milk mixture evenly over the layered ingredients, pressing gently so the bread soaks up the custard. Sprinkle with chopped chives or parsley if using.

8 Let the casserole sit for 10–15 minutes so the bread absorbs the custard (or cover and refrigerate for up to overnight for a make-ahead option).

9 Bake uncovered in the preheated oven for 35–40 minutes, or until the top is golden and the center is set (a knife inserted in the center should come out clean).

10 Remove from the oven and let rest for 5–10 minutes before slicing. Serve warm alongside a simple green salad or fresh fruit.