Place the potatoes in a pot and cover with cold water. Gentle simmer until very tender.
While the potatoes cook, place the oil in a medium pot set over medium heat. Add the turkey and cook and stir until cooked through and crumbly. Drain any excess liquid from the pot, and then mix in the onion, carrot, garlic, flour and sage. Cook and stir 3 to 4 minutes more. Add the tomato sauce and stock, bring to a simmer, and simmer 2 minutes. Mix in the peas, corn, salt and pepper, and then remove from the heat.
Divide the turkey mixture between 6 small (about 1½ cup) baking dishes or tinfoil pans, filling each about 2/3 full.
Preheat the oven to 350 degrees F. When tender, drain the potatoes well. Thoroughly mash, and then beat in the butter and milk. Season the potatoes with salt and pepper. Divide and spread potatoes over the turkey mixture. Bake 30 minutes, or until golden and bubbly.
Note: Sage leaves are sold in bottles in the herb and spice aisle. It is crumbly looking, not finely ground.