Cook the macaroni according to package instructions, except cook for about 1 minute less. Drain but save about ¼ cup pasta water.
16 ounces elbow macaroni
In the meantime, heat the cast iron skillet over medium heat, then add the olive oil to the skillet. Whisk the flour into the olive oil and cook for about 2 minutes, whisking until there are no clumps.
¼ cup olive oil, ¼ cup flour
Slowly add the milk, heavy whipping cream, cream cheese, and ground mustard, stirring to combine. Bring the mixture to a simmer for about 3-5 minutes, until it starts to thicken, and then reduce the heat to low.
½ cup heavy whipping cream, 2 cups milk, 4 ounces cream cheese, 1 teaspoon ground mustard
Begin to add the cheddar cheese and queso fresco, stirring to make sure they fully melt. If the sauce seems too thick, add some of the ¼ cup of pasta water that was saved from draining the pasta.