Preheat the oven to 375°F and lightly grease a 12-cup muffin tin.
Place the potatoes in a pot of salted water and bring to a boil. Cook until tender, about 15 minutes, then drain. Mash with the butter and milk until smooth. Set aside.
Heat a skillet over medium heat. Add the onion and garlic and cook until softened. Stir in the spinach and cook until wilted. Remove from heat and let cool slightly.
In a large bowl, combine the ground lamb, feta, oregano, breadcrumbs, egg, salt, pepper, and the cooled spinach mixture. Mix until just combined.
Divide the mixture evenly among the muffin cups, pressing it down lightly. Spoon the mashed potatoes over the top of each mini meatloaf and spread to cover.
Bake for 25 minutes until the meat reaches 160°F internally. Let rest for 5 minutes before serving.