In a frying pan, add oil and brown medallions. You may have to use more than 1 tablespoon of oil depending on how many medallions you get. They do not need to be fully cooked through as they will be finished off in the oven. They should have a golden brown color in their searing process. Remove from pan and place into a deep casserole dish or dutch oven or roaster.
Pour half of the water into the frying pan to de-glaze the fry of medallions. Make sure you remove caramelization as you stir and bring water to boil.
Add to this 1 cup of the heavy cream. Then add garlic and bring to boil; reduce heat immediately.
In a separate cup, use the remaining cold water and stir in your cornstarch, mix well and then add to your cream mixture in frying pan. Stir until thickened. Take off heat and pour over medallions, scraping out the pan.
Rinse pan if heavily coated in cream mixture, then melt butter and add in the onion and mushrooms. Gently sauté until mushrooms are tender. Add in remainder of cream. Take off heat and pour over medallions.
Cover and bake for 2 hours.
This is great just with a salad… but better served over top one of the following: mashed potatoes, quinoa, rice or pasta. Enjoy!