Preheat oven to 425°F.
Using a sharp knife, cut slits into the potatoes that are approximately ⅛ of an inch apart and do not cut all the way through (you can use chopsticks or something similar to brace each side of the potato and stop your knife from going through).
Place cut potatoes in a bowl. Mix with olive oil and plenty of salt and pepper.
Place cut side up onto a parchment covered roasting pan.
Roast for approximately 60 minutes, or until potatoes are tender and golden brown on the outside.
Meanwhile, add dill, scallions, salt, pepper, lemon zest, and lemon juice to a food processor and pulse until finely chopped. Add the sour cream and mayonnaise and pulse until combined. (alternatively, you can just chop the dill and onions finely and mix in a bowl)
Serve the potatoes warm with the dip.