Preheat oven to 425°. In a medium bowl, toss potatoes with 1 Tbsp. oil to coat; season generously with salt and pepper. Spread potato halves in a single layer on a rimmed baking sheet. Roast potatoes, turning occasionally, until tender and golden brown, 25-30 minutes.

Meanwhile, combine 1/2 cup chives, parsley, almonds, and garlic in a processor. Pulse until finely chopped. With machine running, gradually add remaining oil through feed tube and process until incorporated. Transfer chive pesto to a small bowl. Stir in lemon juice, then 2 Tbsp. water. Season pesto with salt and pepper.

Transfer potatoes to a platter. Drizzle with half of pesto; sprinkle with additional chopped chives. Serve with remaining pesto.