Preheat the oven to 350 degrees Fahrenheit.
Heat the oil in a 12-inch cast iron skillet. Add the garlic and onions, season with salt and ground black pepper, and cook over medium heat until soft but not brown.
Add the sausage and stir for a couple of minutes until it starts to brown.
Add the mushrooms and stir well. Saute for a few minutes until the mushrooms start to soften, then add the sage or thyme and the flour and mix well. Cook, stirring, for another couple of minutes.
Add the frozen veggies, mix, and then add the nondairy milk. Stir frequently until the mixture just comes to a boil. Add more salt and pepper as needed. Turn off the heat.
Arrange the frozen tater tots on top of the veggies. You can make a simple but appealing design, like I did, by arranging the tots in concentric circles.
Place the skillet in the oven and bake 55 minutes or until the tater tots are golden-brown.
Remove the skillet from the oven and let stand 5 minutes before serving.