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 1/2 cup green onion. Cook until no pink remains, seasoning with 1/2 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 1/4 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.