In a small bowl, mix the mustard, salt, and pepper.
Add just enough olive oil to make a paste.
Place the fillets skinned-side down and divide the paste between the two, rubbing into the side facing up.
Place a nonstick skillet over medium-high heat.
When hot, add some oil.
Place salmon in the skillet, mustard-coated side down.
Sauté for 2-3 minutes until golden-browned.
Turn, reduce heat, and cover, allowing salmon to cook all the way through - another 5-7 minutes, depending on the thickness of the fillets.
To plate, reheat the sorrel sauce (don't let it come to a boil) and divide among two plates. Top with salmon and serve.