Preheat oven to 375 degrees F.
Meanwhile, heat oil in a large skillet over medium heat.

Add onion and sauté until soft, about 4-5 minutes.

Add ground beef, breaking the meat up with a wooden spoon. Season with salt and pepper, and cook until browned all over and cooked through, another 4-5 minutes.

Turn off heat and drain fat.

Return skillet over low heat and check for seasonings, adding more salt and pepper if necessary.

Add ketchup and mustard and stir until evenly combined. Turn off heat.