Step 1

Arrange a rack in the middle of the oven and heat the oven to 400°F. Meanwhile, line a rimmed baking sheet with aluminum foil, and place 6 strips bacon in a single layer on the baking sheet. Thinly slice 1 medium red onion and finely chop until you have 3 tablespoons fresh chives, and place both in a large bowl.

Step 2

Roast the bacon until browned and crispy, 15 to 20 minutes. Set aside to cool for 5 minutes. Transfer the bacon to a cutting board, chop into bite-sized pieces, and add to the bowl with the onion and chives.

Step 3

Carefully transfer 2 tablespoons of the rendered bacon fat to a medium bowl. Do not discard the remaining bacon fat or remove the foil from the baking sheet. Add 1 1/2 pounds small new potatoes or baby red potatoes to the baking sheet and toss to coat them in the fat. Season with 1 teaspoon of the black pepper and 1 teaspoon of the kosher salt and carefully toss to distribute seasoning. Roast until fork tender, 20 to 25 minutes. Meanwhile, make the dressing.

Step 4

Prepare the following, adding them to the bowl of reserved bacon fat: Squeeze the juice from 1 medium lemon (about 4 tablespoons); finely chop 1 garlic clove and 1 medium shallot. Add 2 tablespoons red wine vinegar, 2 tablespoons whole grain mustard, 2 teaspoons granulated sugar, remaining 1/2 teaspoon black pepper, and remaining 1/4 teaspoon kosher salt. Whisk until the salt and sugar are dissolved and the dressing is emulsified.

Step 5

When the potatoes are ready, remove the baking sheet from the oven. Using the bottom of a measuring cup or drinking glass, press down on each potato until it splits open and is flattened to about 1/2-inch thick. Roast until the bottoms are golden-brown, 10 to 12 minutes. Flip each potato over and continue roasting until the bottoms are golden brown and the potatoes are crispy all over, 10 to 12 minutes more.

Step 6

Transfer the potatoes to the bowl of bacon and onions. Add 2 cups arugula and 1/4 cup of the dressing, and gently toss to combine. Taste and season with more dressing, salt, and pepper as needed. Serve immediately.