How to Make It
In a small saucepan, cook the shallots in the olive oil over low heat, stirring frequently, until softened but not browned, about 10 minutes. Transfer the shallots and oil to a blender and let cool briefly. Add the vinegar and mustard and puree until smooth. Add the parsley and pulse until finely chopped. Season the vinaigrette with salt and pepper.
Meanwhile, light a grill or preheat a grill pan. Season the steaks with salt and pepper and grill over moderately high heat, turning once, until medium-rare, 10 to 12 minutes. Let the steaks rest for 5 minutes. Transfer to plates, top with the vinaigrette and serve.