Add all ingredients to a screw-top jar. Lid and shake well.
FOR THE SALAD
Toss the arugula, fennel, and red onion together in a salad bowl. Serve with balsamic dressing and blue cheese crumbles on the side. Add only enough dressing to lightly coat each serving of salad (I drizzle on just about a tablespoon for mine).
This recipe makes extra dressing. Reserve remaining dressing in a jar in the refrigerator for up to 2 weeks.
Refrigerate leftover salad in an airtight plastic container and eat within 3–4 days.