Slice the potatoes into matchsticks, toss with olive oil, salt, and pepper. Spread on a baking sheet and bake for 35-45 minutes, flipping halfway to ensure even crispness.
Meanwhile, heat a skillet over medium heat. Add the remaining olive oil and sauté 1 cup diced yellow onion until soft.
Add lean ground beef, Dijon mustard, ketchup, and season with salt and pepper. Cook until the beef is browned and crumbly.