Crispy Smashed Potatoes with Green Horseradish Sauce

Combine potatoes, bay leaves and rosemary in a large pot and cover with 3 inches of water. Bring to a simmer over high heat. Reduce heat to medium-low, partially cover and simmer until the potatoes are just tender, 30 to 40 minutes. Drain (discard the bay leaves and rosemary) and let the potatoes stand until cool enough to handle, about 15 minutes.

Position a rack in lower third of oven; preheat to 450 degrees F.

Combine butter and oil in a small bowl. Pour half onto a large rimmed baking sheet. Add onion and toss to coat. Halve the potatoes lengthwise and place them cut-side down on the baking sheet. Smash the potatoes gently with the bottom of a jar. Brush with the remaining butter mixture and sprinkle with 1/4 teaspoon salt and 1/8 teaspoon pepper.

Roast the vegetables until browned on the bottom, 25 to 30 minutes. Turn over and season with 1/4 teaspoon salt and the remaining 1/8 teaspoon pepper. Roast until dark golden brown, 15 to 20 minutes more.

Meanwhile, to prepare sauce: Combine parsley, chives, dill, horseradish, garlic and salt in a food processor. Process until finely chopped. Add sour cream, yogurt, lemon zest, lemon juice, coriander, sugar and cayenne; pulse until smooth.

Serve the potatoes with the sauce.