Season the sole with salt and freshly ground black pepper.
Heat a frying pan until hot and add a third of the butter to the pan.
Place the fish into the pan and cook for two minutes on each side. Set aside and keep warm.
Meanwhile, heat a separate frying pan and add another third of the butter to the pan, along with the baby onions.
Fry the onions until golden-brown on all sides and tender - about five minutes.
Heat another frying pan until just hot and add the remaining butter, shallots and thyme.
Cook for two minutes, then add the white wine. Turn up the heat, and cook until reduced by half.
Add the fish stock to the pan and allow to reduce a little further. Add the cream and simmer for 3-4 minutes.
Strain the sauce through a sieve and return it to the pan. Add the peas and sorrel and cook through until the sorrel is wilted and the peas are hot.
To serve, place some sauce on each plate and arrange the fish on top. Spoon the onions around the edge and serve at once.