Step 2) Heat oil in a large skillet over medium-high heat, add onions and cook for 5 to 8 minutes until tender and beginning to brown. Reduce heat and add garlic; cook for 1 minute. Transfer onion mixture to electric blender or food processor. Add chipotle chilies and chicken broth. Puree until smooth. Pour over chicken in slow cooker; cover. Cook for 2 to 3 hours on HIGH, stirring once, cooking until chicken is tender and at least 170℉.