Rinse the chicken breast fillet under cold running water and pat dry. Melt half of the butter in a saucepan and heat the flour in it, stirring, until it is light yellow. Gradually add the stock and cream while stirring, whisk through with a whisk, being careful not to create any lumps. Bring the sauce to the boil. Add the spinach to the sauce without defrosting and heat it, stirring occasionally. Add the horseradish to the sauce, bring to the boil, season with salt and pepper and let cool down a little. Finally fold in the egg and pour the sauce into a baking dish.
Season the chicken breast fillet with salt and pepper, place in the sauce and sprinkle with breadcrumbs. Spread the remaining butter on top in flakes. Put the baking dish in the oven.
Bake in the convection oven at 180 degrees for 20-25 minutes