Place the sweet potatoes, potatoes and whole garlic clove in a large pot of salted water. Bring to a boil and cook for 12 to 15 minutes, or until tender. Drain, return to the pot and set aside.
While the potatoes are cooking, heat 1 Tbsp oil in a large, deep skillet over medium-high heat. Add the onion and sauté for 5 to 6 minutes, or until transparent and soft.
Add the chopped garlic, carrot and celery. Reduce the heat to medium and sauté for 5 minutes.
Stir in the peas and cook for 1 minute.
Remove the vegetables from the skillet, transfer to a small bowl and set aside. Wipe the skillet clean with a paper towel and return to medium-high heat. Heat the remaining 1 Tbsp oil.
Add the ground turkey, ginger, 1 tsp salt, 1/4 tsp cayenne, cumin and cinnamon and cook, stirring, for 5 minutes.
Sprinkle on the flour and cook, stirring, for 2 minutes, or until the turkey is cooked through and the juices have thickened.
Return the vegetables to the skillet, combine with the turkey and remove the pan from the heat.
Preheat the oven to 400°F.
Place the pot of drained potatoes over low heat. Add the warm milk, yogurt, the remaining 5 tsp salt and 4 tsp cayenne. Mash together with a hand masher or hand-held mixer.
Spoon the turkey mixture into a 13- by 9-inch baking dish. Spread the potato mixture on top in an even layer. Bake for 20 minutes, or until the top is golden brown.