Arrange the potato tots in a single layer on a baking sheet. Sprinkle with the taco seasoning. Bake according to package instructions. Let cool on the baking sheet for 5 minutes.
Transfer the potato tots to a large platter, stacking them no more than two high. Pour the Perfect Cheese Sauce over the tots. Garnish with the tomatoes, jalapenos, scallions, cilantro and avocado. Sprinkle with the lettuce and bacon, if using.
Melt the butter in a medium saucepot over medium heat. Stir in the flour and cook, stirring often, until a thick paste forms and turns golden brown. Stir in the taco seasoning and cook 30 seconds more.
Gradually whisk in the milk, adding 1 cup at a time. Allow the mixture to come to a simmer before adding the next. It will be extremely thick at first and will thin with each addition. When all the milk is added bring the sauce to a simmer and cook 5 more minutes. Whisk in the cream cheese.
Remove the sauce from the heat. Add the Cheddar and Parmesan and whisk until smooth. The sauce keeps refrigerated for up to 2 weeks or frozen for 6 months. If using from cold, carefully bring the sauce back to a boil before serving.