I boiled one chicken breast in three litres of water with three medium-sized raw onions, two teaspoons of ginger powder, two teaspoons of dried garlic flakes, 10–15 whole black peppers, two table spoons of canola oil, and three teaspoons of salt. After boiling for almost 35 minutes, the water is reduced to two litres. Then I added three coils of egg noodles and cooked until the noodles were soft. I tried the noodle soup, and it tasted bland. It felt like this soup was missing a lot of things. I finished eating the noodles, but I still have the breast piece and almost one litre of soup. What can I add more to improve the taste of this soup? N.B. I am allergic to bell peppers or chili peppers. 
Based on your recipe, things I would change:  Instead of boiling, bring the water to a boil, then immediately drop to a simmer. Cooking things too hot too fast almost always results in degradation of flavor, and in a soup you generally need a long cook time to get the flavors to diffuse and blend properly, so you need a lower temperature for cooking. Gently sauté or roast the onion before using it in the soup. Ideally, do this in the bottom of the pot you’re cooking the soup in so you get all the flavors in the stock. This will help bring out the flavors more, as well as making the onions softer in the final soup.  Possibly do the same with the chicken. Unless you particularly dislike garlic, use at least another teaspoon, possibly more. A number of the compounds in garlic (or produced as part of cooking with it) act as flavor enhancers, much the same way that salt does. Grind the spices using a mortar and pestle before adding them. The higher surface area will transfer a lot more flavor and also help the flavor diffuse better throughout the soup. Consider using more chicken. One breast is not a lot for that much soup, so the flavor from it is not going to be particularly strong. This is especially the case if the breast is boneless, as soaking the bones is a large part of what adds flavor to meat broths. Alternatively, consider using bouillon cubes (chicken bouillon would be the natural choice here) or even instant dashi (if you are going for an East Asian flavor) to add to the flavor without adding a lot of extra calories. Consider using an oil other than canola oil. Canola oil has very little flavor, which makes it useful in some cases (for example, as a carrier for flavors that don’t blend well with water), but not as great in others. Depending on the exact flavor you’re going for, I would consider olive oil, sesame oil, peanut oil, or even coconut oil. Consider using more vegetables. Top of the list for me would be celery and/or carrots (probably diced and sautéed alongside the onions, so you get something a bit like a mirepoix as a base). Both are flavorful vegetables that work well in soup. Other possibilities that come to mind include tomato (either as a paste or finely diced, added later on), daikon (will add some texture because it stays crunchy even when cooked), or some variety of mushrooms (I would probably use shitake or boletes).  