In a large saucepan, melt 1 tablespoon butter over medium heat. Add onions and cook, stirring until soft. Add sliced mushrooms, thyme, salt, and pepper and another tablespoon of butter.
Cook, stirring often, until the mushrooms and onions start to brown, about 5 to 10 minutes.
Add the third teaspoon of butter and stir until melted. Add the flour and cook, stirring constantly until the mushroom and onion mixture is evenly coated. Add one cup of chicken stock at a time, stirring between additions and stir until the mixture boils. Reduce heat to low and simmer uncovered, stirring occasionally, for about 15 minutes.
Add cream and cook, stirring frequently, for about 5 minutes.