Shell and devein the shrimp. Reserve the shells for the stock (see note). Poach the shrimp in a large saucepan over low heat in white wine, chicken stock, or water to cover until cooked through, 4 to 6 minutes.
Melt the butter in a large saucepan over medium heat. Add the onions and cook until translucent, 10 to 15 minutes. Add