In a medium salad bowl, toss the Brussels sprouts, pecans, cheese and parsley. Add the vinaigrette, toss, season, and plate.
Put the mustard in a small heavy bowl that won’t move around when you start whisking. Slowly whisk in the olive oil to emulsify. If it gets too thick, dilute with a touch of water. Once the oil is incorporated, add the lemon juice and vinegar, season to taste and finish with the parsley.