Directions

• Step 1 Put potatoes, bay leaves and rosemary in a large pot with enough water to cover the potatoes by about 3 inches. Bring to a simmer and simmer (partially covered) for about 40 minutes until potatoes are tender. Once potatoes are cooked, remove from pot and allow to dry for about 15 minutes until potatoes are easy to handle. Alternatively, steam potatoes on a rack in an Instant Pot.  
• Step 2 Preheat oven to 450 degrees F.
• Step 3 Combine butter and oil. Pour half of the butter mixture into a large rimmed baking sheet. Slice onions into thick slices, place on the baking dish and toss with the melted butter. Slice the potatoes in half lengthwise and place face down on the baking sheet. Use the bottom of a large glass or bowl to gently smash each potato. Brush smashed potatoes with the remaining butter.  Sprinkle with salt and pepper. Roast potatoes until browned on the bottom for 25 to 30 minutes.
• Step 4 After 25 to 30 minutes of roasting, flip the potatoes over and continue roasting for about 15 more minutes until they are golden brown on the top. Be careful not to let the onions (or potatoes) burn.
• Step 5 Prepare the sauce. Combine parsley, cilantro, 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. Mix until smooth
• Step 6 Spread room temperature sauce on the bottom of a serving dish. Arrange the smashed potatoes over the top of the sauce.Garnish.