Melt the butter and add the flour. Pour in the vegetable stock and stir well until everything is dissolved. Add the cream and bring everything to the boil. Add the spinach leaves that have now thawed and season with horseradish, salt and pepper. Add the egg at the end, but don`t let it boil anymore.
Spread the sauce in a baking dish or on a baking sheet. Season the chicken breast fillets with salt and pepper and place on the sauce. Now sprinkle the fillets with the breadcrumbs and put flakes of butter on top.
Bake in a preheated oven at 180 ° C for about 40 minutes.