To prepare the potatoes: Put the potatoes in a large saucepan of salted water and bring to a boil over medium-high heat. Cook until tender, 15 to 20 minutes. Drain in a colander. Put the potatoes through a ricer or food mill placed over the saucepan. Add the butter, sour cream, and pressed garlic, and mix until smooth and fluffy. Set aside.
To prepare the lamb filling: In a large skillet, heat the oil over medium heat and sauté the onion, stirring occasionally, for 10 minutes. Add the lamb, using a wooden spoon to break up the meat. Continue to sauté for 5 minutes. Stir in the minced garlic and continue to cook until the meat is browned. Drain off the fat.
Sprinkle the flour over the meat mixture and cook, stirring, for 2 minutes. Add the broth, thyme, chopped oregano, rosemary, and nutmeg. Bring to a boil, reduce the heat to a simmer, and cook for 10 minutes. Taste and adjust the seasoning with salt and pepper. Remove from the heat and let cool slightly.
To assemble the pie: Preheat the oven to 350°F. Divide the lamb mixture among 4 single-serving ramekins, or place in a 9-inch baking dish. To create the crust, spread the mashed potatoes over the lamb, forming small peaks. Bake until the potatoes are flecked with brown, about 30 minutes. Garnish with oregano sprigs, if desired.