In a large saute pan with high sides, melt 3 tbsp of butter over medium heat.
Add roughly chopped mushrooms and diced onion, and cook until mushrooms have softened and onions are translucent, 7-8 minutes.
Add garlic, and stir to combine. Cook until garlic is fragrant, 1-2 minutes.
Add 1 tsp dried marjoram, 1 tsp dried thyme, 1/2 tsp salt and 1/4 tsp black pepper. Stir to combine.
Add 1/4 cup of dry white wine to deglaze the pan, scraping up any brown bits off the bottom.
Add heavy cream and chicken stock, stir to combine, and bring to a gentle boil.
Once boiling, reduce to a simmer for 2-3 minutes to let the flavors come together, stirring often. Remove from heat.
Using a blender or immersion blender, blend the sauce until very smooth, or to your desired consistency. (BE CAREFUL if using a blender that you don't put the sauce in while it is too hot, or you will risk the steam building up and creating a dangerous, messy explosion all over your kitchen). Transfer sauce to a large bowl and set aside.