Steam potatoes (using a steamer insert) over high heat until you can easily break them with a fork, about 20 to 25 minutes.
Whisk lemon juice, Dijon mustard, garlic, sea salt and pepper together. Slowly add olive oil while whisking to emulsify.
Add hot potatoes to a bowl and toss with dressing until well coated.
Cover and let sit at room temperature for a few hours. If you’re making it more than a few hours in advance, refrigerate it and bring it out an hour before serving.
Just before serving add basil and a bit more dressing, if needed.
Notes
* If you're using larger potatoes you'll want to cut them into at least quarters. * You may have extra dressing – use it as a dressing for a green salad. * Dressing keeps for a few weeks in the refrigerator. * Recipe can easily be cut in half or doubled.