Step 2
Melt butter in a 12-inch cast-iron skillet over medium-high. Add onion, and cook, stirring, until just beginning to brown, 4 to 5 minutes. Add flour and garlic to skillet; cook, stirring, until fragrant, 1 to 2 minutes. Add milk; bring to a boil. Reduce heatto low. Gradually add Cheddar and processed cheese, whisking until melted. Remove from heat. Stir in cooked pasta, mayonnaise, salt, and 1/2 cup of the parsley.