Watercress Sauce

2 tbsp olive oil
2 tbsp chopped shallot
1 garlic clove, minced
1 tbsp white wine vinegar
1 large bunch watercress
Salt and freshly ground pepper
water, if needed

In a small saucepan, heat the oil over medium heat. Add the shallot and garlic and cook until softened, about 3 minutes. Remove from the heat and stir in the vinegar. Set aside.
In a medium saucepan of boiling water, blanch the watercress for about 2 minutes. Run under cold water and drain well.
Transfer the watercress to a blender, add the shallot mixture and process until smooth. Season with salt and pepper to taste. Add a small amount of water if the sauce is too thick.
Return the sauce to the saucepan and heat over medium heat until hot. Serve immediately.