Arrange chicken breasts in the bottom of the crock-pot. If large, cut int half. Add onions.
In a medium bowl, whisk together honey, soy sauce, hoisin, ginger, olive oil, garlic, and red pepper flakes. Pour sauce over chicken and onions. Cover crock-pot and cook chicken on LOW HEAT for 3-4 hours or on HIGH HEAT 1½-2 hours, until the chicken is fully cooked. (cook time depends on your crock-pot. Mine took 1½ hours on high heat).
Using tongs, remove chicken from crock-pot (reserving the liquids) and place it on a cutting board. Use two forks to shred chicken into smaller pieces or slice chicken into thin slices and shred with your fingers. Place shredded chicken in a large bowl.
In a very small bowl, whisk together 1 tablespoon flour with 2 tablespoons water. Mix well to dissolve. If you using cornstarch, use ½ tablespoon cornstarch and mix with 1 tablespoon of cold water. Mix well to dissolve.
Transfer liquids from crock-pot into a small saucepan set over medium-high heat and whisk in flour or cornstarch. Bring sauce to a boil and cook until it reduces and has thickened slightly about 4 minutes. Pour sauce over chicken, tossing to combine.
Serve chicken over steamed shredded, cabbage, or brown rice, if desired. Check Serving Tips, below. Top with sliced scallions, if desired.
Any leftovers freeze great.