Heat 1 tablespoon oil in a large cast-iron skillet over medium heat.
Add diced onion and minced garlic; cook until softened, 2 to 3 minutes.
Add ground beef; cook, stirring occasionally and breaking it apart, until no longer pink, 2 to 3 minutes.
Stir in diced sweet potatoes, chopped bell pepper, ½ teaspoon salt and ¼ teaspoon pepper.
Cover and cook, stirring occasionally, until the sweet potatoes are tender, 10 to 12 minutes.
