80g finely grated Gruyere cheese (you can use a sharp cheddar here if preferred)
1/2 teaspoon salt
1/4 teaspoon freshly ground black or white pepper
Pinch of freshly grated nutmeg
80g fresh breadcrumbs, preferably whole-wheat
Salt and freshly ground black pepper to taste
Preheat oven to 200°C. Put a large pot of water on to boil. Coat a 1 1/2-quart gratin dish or similar-size shallow baking dish with cooking spray.
Cook kohl rabi in the boiling water until tender, 20 to 25 minutes. Drain.
Heat 1 cup milk in a small saucepan over medium heat until steaming. Whisk flour and the remaining 1/3 cup milk in a small bowl to make a smooth paste; stir into the hot milk and cook, whisking constantly, until the sauce bubbles and thickens, 2 to 3 minutes. Remove the pan from the heat and stir in ham and cheese. Season with salt, pepper and nutmeg.
Distribute the cooked kohl rabi in the prepared dish. Pour the cheese sauce over the top, spreading evenly. Sprinkle with breadcrumbs.
Bake the gratin until bubbling and golden on top, 30 to 40 minutes.