Pre-heat the oven to 375 F.
In a medium bowl, mix turkey, garlic, sage, green onions, egg, pepper, and one teaspoon salt.
Using a 10-inch cast iron skillet, heat the olive oil and cook the turkey until it reaches an internal temperature of 165 F, then place it in a 9-by-13-inch baking dish.
Blanch the green beans by boiling them for 2-3 minutes, then cooling them in ice water and mixing them with the turkey in the dish.
Fry the bacon until crispy, chop into ¼-inch pieces, and add to the turkey mixture.
Heat 2 tablespoons of butter, add mushrooms and cook until browned, then add to the casserole dish.
In a saucepan, melt the remaining butter, add flour and cook for 2 minutes, then whisk in the milk and half and half. Add onions, salt, and pepper. Stir in 2 ½ cups of cheese until melted.
Pour the cheese mixture over the ingredients.
Scatter tater tots over the top and add the remaining cheese.
Bake for 45 minutes or until golden brown.
