Whisk mustard, mayonnaise, oil, vinegar, dill and salt in a medium bowl. Transfer 3 tablespoons of the dressing to a large bowl. Add beans, tuna and onion to the remaining dressing in the medium bowl; toss gently to coat.
Add spinach and beets to the dressing in the large bowl. Toss gently to coat.
Serve the tuna mixture over the spinach salad. Sprinkle with fresh dill and/or pepper, if desired.
