Heat a non-reactive pan over medium heat and add a splash of olive oil. Swirl to coat, then add the fennel and whole onions. Brown each side of the onions being careful not to burn the fennel.
Add the wine and allow it to boil until there is almost no liquid left. This burns off the alcohol and allows the flavor of the wine to concentrate before we add more liquid.
Add the chicken stock and continue to boil until the liquid is reduced by half.
While the sauce is reducing, rub the skate with olive oil then lightly salt and pepper both sides.
Heat a cast iron skillet skillet over medium high heat until very hot. Lay the fillets in the pan and allow them to lightly brown on one side then use a thin long spatula to flip them over.
When the sauce has reduced by half, whisk in 2-3 tablespoons of butter then salt and pepper to taste. Depending on the type of wine you used, the sauce may be a little tart. By adding honey you can balance out the the tartness with the sweetness from the honey. Finish the sauce by adding the tarragon and a squeeze of lemon.
Plate the skate fillets then top with the braised onions, fennel and sauce.