Preheat the oven to 400° F.

While oven is preheating, heat a large pot of lightly salted water over high heat and prepare the pasta according to package directions, cooking 1-2 minutes less than al dente. (Pasta will continue to cook during baking). 

Meanwhile, in a large oven safe skillet, heat olive oil over medium heat. Add in the onion and season to taste with salt and pepper. Sauté, stirring occasionally, until the onion becomes soft and translucent, approximately 3-4 minutes.

Sprinkle flour over the onion and cook, stirring constantly, for 1-2 minutes. 

While stirring constantly, slowly stream the milk and cream into the onion and flour mixture. Let the milk come to a gentle simmer and continue cooking and stirring until the béchamel begins to thicken, approximately 3-4 minutes. 

Stir in the Italian seasoning, cheddar, and one-half of the fontina cheese.

Continue cooking and stirring until the cheeses melt.   

Drain the cooked pasta and combine with the cheesy sauce. Remove from heat and top with marinara, pepperoni slices, and reserved fontina. 

Transfer the skillet into the oven and bake until the cheese is all melted and bubbly, 15-20 minutes. Enjoy!