To make the dressing: Combine the olive oil, balsamic and mustard in a jar with an airtight lid. Add a pinch of salt and black pepper. Screw on the lid and shake vigorously. This vinaigrette will keep on the counter for several weeks (refrigerate if you added any fresh ingredients). The oil and vinegar will separate a few minutes after shaking — shake to recombine before dressing your salad.