1. Preheat the oven to 180°C (350°F).
2. Cut the broccoli into small florets and remove the stalk, then cook the florets in salted water for 5 to 10 minutes in a pressure cooker, then drain them.
3. In a saucepan, melt the butter, then stir in the flour before it takes on color, stirring vigorously.
4. Off the heat, pour in the milk while whisking and return to the heat while stirring until thickened, then remove from the heat and add the cream, egg, nutmeg, and 80 g of grated Gruyère cheese.
5. Butter a gratin dish, arrange the broccoli, pour the sauce over it, sprinkle with grated Gruyère cheese, then bake for 15 minutes and finish by placing the dish under the broiler to brown.
6. Serve the broccoli gratin.