Preheat the oven to 400F (200C).

Make the cauliflower crust: steam or boil the cauliflower florets until very soft, about 10 minutes, then drain well and let them steam dry for a couple of minutes. Mash with the cream cheese, butter and horseradish (if using) until smooth, season with salt and pepper, and set aside.

Make the filling: heat the olive oil in a large skillet over medium-high heat. Add the chicken pieces, season with salt and pepper, and cook for 6-8 minutes, stirring now and then, until cooked through. Add the baby spinach and cook for another minute or two until fully wilted.

Use a slotted spoon to transfer the chicken and spinach to two 12 oz baking dishes, leaving any liquid behind in the pan.

Make the sauce: add the cream cheese, heavy cream and water to a small saucepan and heat over low heat, stirring, until the cream cheese has melted. Whisk a spoonful of the hot sauce into the egg yolk, then pour the yolk back into the pan and stir constantly over low heat for 2-3 minutes until slightly thickened. Season generously with salt and pepper.

Divide the sauce between the two dishes, then spoon the cauliflower mash over the top and spread it to the edges. Rough up the surface with a fork.

Set the dishes on a baking sheet to catch any drips and bake for 15-20 minutes, until the topping is golden brown in places.

Let the pies rest for 5 minutes before serving; the sauce thickens as it cools.