In a heavy bottom pot, melt the butter and oil over medium heat. Add the sliced onions and cook until the onions start to soften, stirring frequently, about 10 minutes. Add a pinch of salt and sugar. Reduce the heat to medium-low and continue cooking until the onions turn golden brown, another 20-25 minutes. Stir the onions occasionally and add one or two teaspoons of water if they start to dry out. Transfer the onions to a plate and set aside.
Preheat oven to 400 degrees.
Using the same pot set over medium heat, add the pork sausage and cook until browned, breaking the sausage apart as it cooks. Drain the sausage on a paper towel-lined plate. Set aside.
In a large bowl or pitcher, add the eggs, heavy cream, french onion dip, and salt and pepper. Whisk together until well combined. Set aside
In a 9x13 baking dish coated with cooking spray, layer half of the tater tots in the bottom. Top with half of the caramelized onions and sausage, and one-third of the cheese. Repeat the layers with the remaining tater tots, onions, sausage, and another third of the cheese. Pour the egg mixture over the top. Spread the remaining cheese over the top.
Cover the dish with aluminum foil and bake for 30 minutes. Uncover and bake for another 20 minutes until golden brown and bubbly around the edges. Let the casserole sit for 10-15 minutes before serving. Garnish with chopped scallions and chives.