How to Make It
In a medium saucepan, combine the sherry with the sugar and cook over high heat, stirring occasionally, until the sugar is dissolved. Add the rhubarb and cook until just tender, about 1 minute. Strain the rhubarb in a colander set over a bowl to catch the cooking liquid. Return the liquid to the saucepan and boil over high heat until reduced to 1/2 cup, about 8 minutes.
Melt the butter in a medium saucepan. Add the onions and cook over low heat, stirring, until softened, about 1 minute. Add the chicken stock and thyme sprig and a pinch each of salt and pepper. Cover and simmer over low heat until the onions are tender, about 8 minutes.
In a medium saucepan of boiling salted water, cook the fava beans for 2 minutes. Using a slotted spoon, transfer the beans to a bowl and refresh under cool running water. Peel off the tough bean skins and add the favas to the onions.
Heat a cast-iron skillet. Lightly coat the salmon fillets with some vegetable oil and season with salt and pepper. Cook the salmon, skin side down, over moderately high heat until the skin is very crisp, about 4 minutes. Turn the salmon and cook over moderate heat until barely cooked through, about 4 minutes more, depending on the thickness of the fillets.
To serve, discard the thyme sprig and rewarm the onions; add the lemon juice and season with salt and pepper. Bring the sherry-rhubarb liquid to a boil and add the cooked rhubarb. Spoon the rhubarb onto 4 large plates and set the onions alongside. Top with the salmon and serve at once.
Turbinado sugar is available at most supermarkets. Palm sugar is available at Asian markets.