Preheat oven to 350°F. Fill a large stockpot with water. Add potatoes, cover & boil 12-15 minutes or until fork-tender. Drain & set aside to cool.
Meanwhile, in a large skillet, add oil & onion. Sauté 2-3 minutes, or until onions are soft. Add turkey, garlic powder & oregano. Cook 5-7 minutes, crumbling meat, stirring occasionally until meat is cooked through. Stir in mixed vegetables. Mix well.
Add milk to stockpot & whip potatoes until fluffy.
Lightly coat a large casserole dish with non-stick cooking spray. Spread turkey & vegetable mixture on bottom. Carefully scoop & spread potato mixture on top of turkey. Top with cheese & bake 10-15 minutes.
Let cool 10 minutes & serve.
For a different flavor, substitute the oregano with thyme or rosemary!