Preheat Oven: Preheat your oven to 375°F (190°C).
Prepare Turkey Mixture: In a mixing bowl, combine ground turkey, chopped garlic, dried sage, green onions, egg, 1 tsp salt, and 1/2 tsp black pepper. Mix until well combined.
Cook Turkey Mixture: Heat 1 tbsp olive oil in a large skillet over medium heat. Add the turkey mixture and cook until browned and cooked through. Transfer the cooked turkey to a 9x13-inch baking dish.
Blanch Green Beans: Bring a pot of salted water to a boil. Add diced green beans and blanch for 2-3 minutes until tender-crisp. Drain and add to the baking dish with the turkey.
Cook Bacon: In the same skillet, cook bacon slices until crispy. Remove, crumble, and add to the baking dish.
Sauté Mushrooms: Drain excess bacon fat, leaving about 1 tbsp in the skillet. Add 2 tbsp butter and chopped mushrooms. Sauté until mushrooms are browned and tender. Transfer to the baking dish.
Prepare Cheese Sauce: In a medium saucepan, melt the remaining 4 tbsp butter over medium heat. Add finely chopped onion and cook until translucent. Stir in flour and cook for 1-2 minutes, stirring constantly. Gradually whisk in milk and half-and-half, cooking until the mixture thickens. Stir in 2 1/2 cups shredded cheddar cheese until melted and smooth. Season with remaining salt and black pepper.
Assemble Hotdish: Pour the cheese sauce over the turkey, vegetables, and bacon in the baking dish. Stir gently to combine. Arrange frozen tater tots evenly over the top. Sprinkle with the remaining 1/2 cup shredded cheddar cheese.
Bake: Place the baking dish in the preheated oven and bake for 45 minutes, or until the tater tots are golden brown and crispy.
Serve: Remove from the oven and let sit for a few minutes before serving. Enjoy this comforting Midwestern classic!