Peel and dice carrot. Peel onion. Peel and cut potatoes in large cube. Wash and add bouquet garni in a casserole dish with a pinch of salt and pepper. Cover with water. Leave to cook on medium heat for 20 minutes.

In a meantime, peel and finely slice shallots. Cook shallots in a saucepan until transparent and deglaze with white wine and vinegar. Add a pinch of salt and pepper. Leave to cook on low heat for 10 minutes.

Reduce heat of potatoes to low heat. Add skate wing and leave to cook for 8 to 10 minutes.

Add diced and cold butter in your sauce. Stir quickly to get a homogeneous sauce. Sieve white butter sauce to remove shallots.

Serve skate wings and potatoes with this delicious white butter sauce.