Slice the potatoes in half lengthwise. The smallest potatoes can be left whole. Add the potatoes to a 9x13 baking dish. Toss with olive oil and season with salt and pepper.
Bake for 40-50 minutes until the potatoes are fork-tender.
Meanwhile, add the arugula, lemon juice, and garlic to the bowl of a mini food processor. Pulse to chop the arugula. Add the olive oil and process until the mixture resembles a thick sauce. Season with salt and pepper.
Spoon the sauce over the potatoes and serve the remainder on the side.
Fingerling potatoes do not require peeling or boiling before roasting.
Store leftovers in an airtight container in the refrigerator for up to 4-5 days. Reheat in the microwave until warm.