How to Make It
Prepare pasta according to package directions; set aside. Preheat oven to 350°F.
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
Stir together ricotta and egg in a small bowl. Gently stir ricotta mixture into pasta mixture, leaving large swirls. Bake in preheated oven 10 minutes.
Meanwhile, stir together panko, melted butter, and remaining 2 tablespoons parsley until blended. Top pasta with panko mixture, and bake until top is lightly browned, about 10 minutes.