Saute shallots (chopped) in white wine and vinegar until they become puree. Slowly add small pieces of chilled butter and stir until everything together becomes a slightly creamy sauce. Season to taste with salt, pepper and lemon.
Nikos Tavridis:
Beurre Blanc is imho one of the best if not the best accompaniment to poached salmon fillet. And you can serve it with lettuce to ease your conscience because of the many calories. I poach the salmon in a mixture of 50/50 fish stock and wine.
Very important: the Sancerre must be good. Who takes a supermarket Sancerre should not be surprised if the sauce does not taste good. And the rest can be enjoyed exquisitely with food.