Bring the water/stock to a boil in a medium saucepan. Put the kohlrabi into the pot, reduce heat, and simmer for 3-5 minutes until tender-crisp. Drain and reserve.

Make the bechamel: Melt the butter in a small saucepan. Heat (but don’t boil) the milk in another small saucepan or in a microwave—the warmer the milk is when you add it to the butter/flour, the faster the sauce will form. Add the flour to the melted butter and whisk vigorously until combined. Keep whisking for at least 2 minutes to cook the flour. Add the milk; if it’s not heated, add a few tablespoons at a time and whisk it in. If you’ve heated it, add all the milk at once and whisk. Continue whisking until a smooth, lump-free sauce forms. Add salt and pepper, and nutmeg if desired.

Fold in the cooked kohlrabi and serve hot.