In a medium-sized saucepan, parboil the potatoes until they are just fork-tender but firm. Allow them to cool to the touch.
Meanwhile, prepare the dressing by combining the Dijon mustard, apple cider vinegar, agave nectar or sugar, olive oil, dill, and salt and pepper. Mix in capers and set the dressing aside.
Grill the potato halves on a grill pan over medium-high heat. Cook about 4 minutes or until they are browned, turn them over and cook the other side. Try to get nice grill marks on them.
Toss the potatoes in the dressing so they are well-coated. Add the red onion slices and chopped celery and toss everything together.
This dressing can also be tweaked to make a creamy, tangy mustard sauce that is great on salad, over baked tofu or tempeh, or over grilled veggies. Mix 3 Tbs. Dijon mustard, 1 Tbs. apple cider vinegar, 1 Tbs. sugar or agave nectar, 2 Tbs. fresh chopped dill and 3 Tbs. extra-virgin olive oil.