Rinse and drain the sorrel. Chop it up and put it aside until later.
Add the chopped shallot, mustard seeds and half of the butter to a medium saucepan. Place this over medium heat and gently cook the shallots in the melting butter for 3 minutes.
Bring the sauce to a simmer and cook for 10 minutes over very low heat. Stir regularly. In the meantime cut the salmon in half. Season both pieces on all sides with pepper and salt. Pour the olive oil in non-stick pan and place it over medium-high heat until hot. Then add the seasoned salmon.
Brown the salmon on all sides until cooked the way you like it.
Remove the salmon from the pan and cover with a piece of tinfoil for now. Add the mustard to the cream sauce.
Stir well and check the seasoning. Add extra pepper, salt or mustard to taste if necessary. Now take the cream sauce off the heat and stir in the drained and chopped sorrel.