Preheat the oven to 180ºC . Butter a 10to 30 cm oval gratin dish. Remove the yolks from the hard-cooked eggs and blend them with the butter, Worcestershire sauce, mushrooms, salt, and cayenne. Spoon the yolk mixture into the egg whites and place them in the buttered casserole. (The dish can be refrigerated up to 3 days at this point.) To make the sauce, melt 3 tablespoons of the butter over low heat. Stir in the flour to make a roux, and cook until lightly browned, about 4 minutes. Add the milk all at once and whisk until thickened and bubbly. Add the Swiss cheese and stir over low heat until melted; do not boil. (The sauce may be made several days in advance and refrigerated, covered with plastic wrap.) Pour the sauce over the stuffed eggs. Melt the remaining tablespoon of butter in a small skillet or microwave. Add the breadcrumbs, and toss to combine. Sprinkle the buttered breadcrumbs over the eggs and bake 20 minutes. Serve at once.
Serves 4