Saute onion in butter until softened, about 3-5 minutes.
Add mushrooms, salt, and pepper. Cook until mushrooms have released juices. Add garlic and thyme and cook until fragrant, about 1 minute.
Add the wine to deglaze and loosen any bits off the pan and simmer a few minutes until the wine has almost evaporated.
Add the chicken broth and cream. Simmer 5 minutes or until reduced by half.
To thicken the sauce further, combine cornstarch with 2 tablespoons of water (or broth) and mix well. Add cornstarch mixture into the broth a little bit at a time to reach desired consistency. You may not need all of the cornstarch mixture.