1. Preheat oven to 375F.
2. Bring potatoes and whole garlic cloves to boil in a pot of water set over high heat. Cook until potatoes are tender, about 15 minutes. Drain well. Mash potatoes and garlic with a potato masher (or your favorite mashing device) until smooth. Add buttermilk and chives and season to taste with salt and pepper. Set aside.
3. While water is boiling, heat 1 tsp oil in a large nonstick skillet over medium-high heat. Add beef or turkey and cook, stirring often and breaking meat into small pieces with wooden spoon. Cook until just done, about 8 minutes. Drain and discard fat, set aside.
4. Heat 2 tsp oil in same skillet over medium-high heat. Add onion, squash, celery and rosemary and cook until soft, stirring occasionally. Add reserved meat, broth and tomato paste and cook until liquid is absorbed, about 5 minutes. Stir in peas.
5. Transfer mixgture to a shallow 2-quart baking dish and cover with mashed potatoes in an even layer. Run a fork over top of mashed potatoes in both directions or swirl with the back of a spoon. Brush top with remaining 1 tsp oilve oil.
6. Bake in oven until filling is bubbling and top is golden brown, about 30 minutes. Let stand 5 minutes before serving.