Place a large saute pan over high heat, add the ground beef and taco seasoning, reserving 1 tablespoon of the mix for the Perfect Cheese Sauce. Mix thoroughly. Cook, stirring often using a wooden spoon or spatula to break the meat into small crumbles. Continue cooking until the meat releases its liquid, and then cook further until all the liquid has evaporated and the meat begins to brown, about 10 minutes.
Combine the Perfect Cheese Sauce and cooked pasta in a large bowl. Pour in the cooked meat and stir to combine. Pour into a 9- by 13-inch baking dish. Level the pasta with the back of a spoon or spatula. Scatter the grated Cheddar over the pasta. Bake until browned and bubbling, 10 to 12 minutes. Remove and let cool 5 minutes.
Immediately before serving, scatter the sour cream, lettuce and fresh tomato over the mac 'n' cheese.
Perfect Cheese Sauce:
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.