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

2 In a large skillet, heat butter or olive oil over medium heat. Add chopped onion and cook until translucent, about 5 minutes. Add garlic and cook 30 seconds more.

3 Add chopped spinach to the skillet in batches, stirring until wilted and any excess moisture has evaporated, about 4–6 minutes. Season with a pinch of salt and pepper. Remove from heat and let cool slightly.

4 In a large bowl, whisk together eggs, milk (and cream if using), Dijon mustard, nutmeg, 1/2 tsp salt and 1/4 tsp black pepper until smooth.

5 Place half of the bread cubes in the prepared baking dish. Sprinkle half of the shredded Gruyère over the bread, then spread the spinach-onion mixture evenly. Top with the remaining bread cubes and remaining Gruyère.

6 Pour the egg-milk mixture evenly over the layered bread and cheese, pressing gently so the liquid soaks into the bread. If time allows, cover and refrigerate for 30 minutes to overnight for deeper flavor and better soak.

7 Bake uncovered in the preheated oven for 30–40 minutes, or until the strata is puffed, set in the center, and golden brown on top. If the top browns too quickly, tent loosely with foil.

8 Remove from oven and let rest 10 minutes before slicing. Garnish with chopped parsley or chives and an extra grating of Gruyère if desired.

9 Serve warm alongside a simple green salad and fresh fruit for a complete brunch spread. Leftovers keep covered in the fridge for 2–3 days and reheat well.