Cut the base off the celery head, separate the stalks, and wash them (tip: add a few drops of vinegar to the water).

Dry the stalks and cut them into pieces.

Peel the onion.

Thinly slice the onion and heat the oil in a large skillet.

Add the onion and sauté until golden.

Add the celery, stir, and cook covered for 5 minutes over medium-low heat.

Wash the peppers, cut them in half, and remove seeds and white membranes.

Cut the peppers into small pieces and add them to the skillet.

Stir and add the horseradish.

Season with salt and pepper, stir, and thaw the broth (tip in link).

Add the broth, stir, bring to a boil, then cover and reduce to low heat.

Layer the ham slices on the work surface and cut them into pieces.

After 30 minutes of cooking, add the ham and preheat the oven to 375°F (190°C).

Stir and add the sour cream.

Mix well to incorporate the cream and add two-thirds of the Parmesan.

Stir and pour into a gratin dish.