Combine the potatoes, garlic and bay leaves in a large pot and cover with cold water by about an inch. Add some salt, then cover with a lid and bring to a boil. Lower the heat and simmer the potatoes until a fork can easily pierce the skin, 15 to 20 minutes.
Preheat the oven to 375 degrees F. Drain the potatoes and return them to the pot with the garlic (be sure to discard the bay leaves). Add the oil and butter and sprinkle with salt and pepper. Using a potato masher, smash the potatoes into a chunky mixture.
Spread the potato mixture in an even layer in a large casserole or gratin dish. Season the potatoes again with salt and pepper, drizzle with additional oil and sprinkle the Parmesan over the top. Bake the potatoes until a crispy, golden crust forms on top, 35 to 45 minutes. Dot sour cream across the top and garnish with dill, chives and lemon zest while hot.