Ask your fishmonger to remove all fish bones from ray fillets. Save the fish bones to make fish stock. Very rich in gelatine, ray bones make excellent fish stock.
Peel two potatoes and cut them in ½ cm slices. Repeat with the sweet potatoes. Slightly grease a cooking sheet with the duck fat and place the potato slices on it while alternating colours.
Season and cover with aluminium foil also greased with duck fat. Bake at 180o Celsius (350o F) in the oven for 20 to 30 minutes. The potatoes must be well cooked and caramelized.
While this is cooking, peel and cook the small onions for 15 minutes on the stove over medium heat in half the fish stock or in equal quantity of water. Cook until the onions are very melted. Drain and lightly colour them in a drop of olive oil in a pan. When they change to a nice golden colour, add a pinch of sugar and save for later with the potatoes.
Wash the chervil and spin-dry it. Place it at the bottom of the mixer, cover with oil and lemon juice, season and start the mixer until it becomes a deep green fin emulsion. Reserve.
Upon serving, place a casserole on the stove over heat with the rest of the fish stock or equivalent water. Cook the ray fillets by putting them in the boiling liquid. When the boiling resumes, reduce heat and cook for 2 minutes. Drain on the plates over the potatoes and small pearl onions that will have been heated in the oven for a few minutes.
Put a spoonful of chervil emulsion on the side.