In a cast iron skillet, sauté the mushrooms until they have released all of their liquid, about 10-12 minutes, stirring occasionally. Remove from the pan and set aside.
Add butter until melted, then cook the onions for 2-3 minutes. Add the garlic and spices, along with salt to taste, cooking for 1-2 minutes.
Whisk together the stock and cornstarch, then stir into the onion mixture. Stir in the heavy cream and add the mushrooms back to the pan. Stir to combine.
Top with most of the cheese, then layer the tater tots evenly. Sprinkle on the last bit of cheese and bake at 375°F for 25-30 minutes until bubbly and golden brown.
