Dissolve bouillon cube in boiling water; set aside.
In large skillet, saute onions in butter, stirring to separate rings and prevent browning.
Stir flour and salt into slightly cooled bouillon; add to onions and cook until thickened.
Combine carrots and onion sauce in crock pot, stirring to coat carrots.
Cover and cook on high setting for 1 hour, then turn to low setting, for 2 to 6 hours.
Before serving, add sugar to taste.