Rosemary Onion Skillet Potatoes

Bahan-bahan (6 porsi)

• 1 tablespoon butter
• 1 large sweet onion (cut into 1/2" pieces, about 1 cup's worth)
• 3 medium russet potatoes (cut into 1/2" pieces, about 4 cup's worth)
• 1 tablespoon olive oil
• 1 tablespoon minced fresh rosemary
• 1 teaspoon kosher salt
• 1/4 teaspoon freshly ground black pepper