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 cooking liquid in a colander set over a bowl, and reserve the rhubarb. Return the cooking 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, cover, and cook over low heat, stirring, until softened (about 15 minutes). Add the chicken stock and thyme 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 or until tender. Using a slotted spoon, transfer them to a bowl and refresh under cool running water. Drain the favas and add them to the onions.

Light a grill or heat a grill pan. Lightly oil the salmon fillets and season with salt and pepper. Grill the salmon over a medium hot fire, skin side down, until the skin is very crisp, about 4 minutes. Turn the salmon and grill over a medium fire until barely cooked through, about 4 minutes more, depending on the thickness of the fish.

To serve, discard the thyme sprigs 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. Cook over high heat, just until hot, about 1 minute. Spoon the rhubarb onto 4 large plates and over the onions. Top with the salmon and serve at once.