Preheat your oven to 400°F.
In a skillet, warm the olive oil over medium heat. Add the diced onions, celery, and carrots. Sauté until they soften slightly, about 3-4 minutes.
Stir in the lean ground beef and cook until browned, breaking it apart as it cooks. Season with salt and pepper.
Mix in the peas and continue cooking for another 2 minutes until the vegetables are tender. Remove the mixture from heat.
Meanwhile, peel (if desired) and chop the small potato. Boil it until tender, about 10-12 minutes.
Drain the potato and mash it with the Greek yogurt until creamy. Season with a pinch of salt and pepper to taste.
Spoon the beef and vegetable mixture into an oven-safe dish, spreading it evenly.
Top the beef mix with the creamy mashed potato, smoothing it out to cover the filling completely.
Bake in the preheated oven for 15-20 minutes until the topping develops a slight golden color.
Allow the dish to cool slightly before serving to enjoy a warm, comforting shepherd’s pie.