Cook the manicotti shells according to the package directions for al dente. Strain and place on a baking sheet to let the noodles cool to room temperature, about 15 minutes. Meanwhile, mix the ricotta, Parmesan, garlic, Italian seasoning, red pepper flakes, 1 egg, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl until smooth. Spoon into a large resealable plastic bag and snip 1/2 inch off of one of the corners.
Push 1 mozzarella stick into the center of each noodle. Pipe the ricotta mixture into the center of each noodle (you may have to pipe into both ends to fill the whole noodle). Place back on the baking sheet.
Set up a breading station with three shallow dishes: Stir together the flour, 1/4 teaspoon salt and a few grinds of pepper in the first dish. Beat the remaining 4 eggs in the second dish and add the breadcrumbs to the third dish. Coat a noodle in the flour, then dip into the egg, letting any excess drip off. Finally, press into the breadcrumbs. Transfer to the baking sheet and repeat with the remaining noodles.
Heat 1/2-inch of oil in a large skillet over medium-high heat and line a plate with a paper towel. Drop a few breadcrumbs into the oil; if they sizzle, the oil is hot enough. Working in batches, fry the noodles, flipping halfway through, until golden brown and the cheese is melted and starting to ooze through the sides, about 4 minutes total. Transfer to the lined plate and repeat with the remaining noodles, making sure to heat the oil back up between each batch. Serve with marinara sauce on the side for dipping.
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