Onions can cause Heinz body formation (denatured hemoglobin within the red blood cell) and anemia. Unfortunately, the only treatment is a blood transfusion or time to allow formation of new red blood cells. Since your little dog is still alive, seems that time will solve this problem.
A liver shunt is a totally different beast. Typical blood results will show low bilirubin, low albumin, high ammonia levels. X-ray films will show a small liver silhouette. Ultrasound may demonstrate abnormal blood flow. Radio-opaque dye is used on some occasions to show abnormal flow, and can be done with MRI, x-ray films and fluoroscopy. Bile acid assays are frequently used to demonstrate liver shunts. Other exotic blood tests are also used, but beyond the scope of this answer. Once a liver is diagnosed, then a surgical repair is the best solution, even though medical treatment may be a viable alternative.
Bottom line answer: if this is onion toxicity, then time will solve your problem over the next 10 to 14 days. If this is a liver shunt, then expect more symptoms and strange episodes as you have described.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124