In 12-inch skillet, heat 1 teaspoon of the oil over medium-high heat. Cook onion and garlic in oil 3 minutes, stirring occasionally, until tender. Transfer mixture to slow cooker using slotted spoon. In skillet, heat remaining 3 teaspoons oil over medium-high heat. Cook chicken in oil 6 to 8 minutes, turning occasionally, until browned. Transfer chicken to slow cooker.