Preheat the oven to 350°F. Spray a 13 x 9-inch baking dish with cooking spray, Set aside.
Drizzle a large skillet with olive oil. Remove casings from sausage and cook over medium-high heat along with ½ cup green onion. Cook until no pink remains, seasoning with ½ tsp garlic salt. Drain all excess fat from the pan set aside.
To assemble: Layer tater tots evenly in the bottom of the baking dish. Sprinkle with 1 cup shredded cheese. Top with sausage and ¼ cup green onion. Sprinkle with 1 cup cheese.
In a blender, whisk together the half & half, eggs, cream cheese, 1 tsp garlic salt, black pepper, mustard and nutmeg. Pour evenly over the filling. Top with the remaining cheese.
Bake for 50-60 minutes until golden and set when gently shaken.
Rest for 5-10 minutes on the counter then sprinkle with the remaining green onion and serve.
It's not necessary to completely thaw the tater tots, just allow them to sit on the counter while you prepare the other ingredients.