This is definitely a crockpot soup. Or I made it that way anyway. Turn your crockpot onto the high setting to get it warmed up.
In a large skillet, heat up oil under medium high heat and cook the chicken medallions until no longer pink. Transfer chicken to crockpot and cover.
In same skillet used for chicken, saute the onions, garlic, and ginger for about 5 minutes, when onions are soft and translucent. Add mixture to crockpot over chicken.
Deglaze skillet of brown bits by adding 1 cup of water to pan and scraping of the sucs. Then dump that water into the crockpot. Add remaining 3 cups of water and 4 cups of chicken broth into crockpot as well. Let simmer in crockpot until almost ready to serve, stirring occasionally. Mine was in there for about 45 minutes.
About 15 minutes before you're ready to dig in, add in noodles and mushrooms. When noodles are soft, stir in rice vinegar, soy sauce, chili flakes, sesame oil, and scallions.