1. Scrub potatoes; place in a pot. Pour in enough cold water to cover by 1 inch. Add a pinch of salt; bring to a boil over high heat. Reduce heat to medium-low; simmer until potatoes are just tender, about 20 minutes.
2. Combine mustard, 1/2 tsp. salt, 1/4 tsp. pepper and lemon zest in a large bowl. Add juice and stir until blended. Slowly whisk in olive oil.
3. Drain potatoes; set aside until cool enough to handle. Using a sharp knife, remove peels. Cut potatoes into 1-inch cubes; add to bowl with shallot and dill; toss gently. Season with salt and pepper, if desired.