Heat oven to 425° F. Spread the fries on a baking sheet. Bake until golden, about 25 minutes.
Meanwhile, cook the ground beef in a medium saucepan over medium-high heat, stirring occasionally, until no trace of pink remains, 8 to 10 minutes. Spoon off any excess fat. Add the marinara sauce and cook until warmed through. Transfer to an 8-by-11-inch or 8-inch-square baking dish. Top with the fries. If desired, return to oven for 5 minutes to allow the flavors to meld.