Preheat oven to 350 degrees. In a large, 12 inch cast iron skillet over medium-high heat, add the ground beef and cook, crumbling as it cooks, until no longer pink, around 5 minutes. Add the onion and continue cooking until lightly translucent, around 4 minutes more. Add the garlic and serrano pepper and cook for 1 minute. Add seasonings and stir thoroughly to combine.
Push the meat mixture to one side of the pan and let the grease drain to the other side. Whisk the baking flour into the grease until a paste forms, then slowly whisk in the beef broth until the paste dissolves and a gravy forms. Stir the meat mixture back into the gravy, then stir in the diced tomatoes and green beans.
In a large plastic bag, add the frozen tater tots, chili powder, garlic powder, and salt. Shake vigorously to coat the tots, then arrange the tots in a single layer on top of the meat mixture in the skillet. Place skillet in the oven for 30-35 minutes, or until the tots are golden and crispy and the meat mixture is bubbling around the tots. For even crispier tots, turn the broiler on during the last couple of minutes. Remove from oven and let sit for 5 minutes before serving.