Preheat the oven to 350°F. Cook the pasta accordingly to package directions for al dente. Drain, toss with a small amount of olive oil to prevent sticking and set aside.
Chop about ½ cup of the pepperoni, leaving 10-12 slices whole.
Melt the butter, adding the flour, garlic and onion powder and Italian seasoning in large saucepan set over medium heat. Whisk together until it forms a paste.
Slowly whisk in the milk. Continue to whisk and bring to a boil. Reduce heat to medium-low and simmer for 2-3 minutes, until it thickens slightly and flavors marry.
Remove the sauce from heat and whisk in the white cheddar cheese.
Toss in the pasta and chopped pepperoni.
Transfer to greased 9 x 13-inch baking dish (or any 3-quart casserole, baking dish or cast iron skillet).
Top with the reserved sliced pepperoni. Spoon marinara sauce on top. Sprinkle with the mozzarella and Parmesan cheese.
Bake uncovered for 15 to 20 minutes or until golden and bubbling.
Remove and allow to sit for 5 minutes before serving. Serve with additional pasta sauce, cheese and chopped parsley, if desired.
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