Heat a stock pot over medium-low heat. Add olive oil, swirl to coat the pan.
Add onions, garlic, and first pinch of salt. Stir until well combined and everything is coated.
Sweat the onions and garlic for a couple minutes.
Add 1/3rd of the mushrooms and a 2nd pinch of salt. Stir well, raise the heat to high. After they have released their liquid, and it has mostly evaporated stir in another 1/3 of the mushrooms and the 3rd pinch of salt. Stir in the bundle of thyme. Again, once the liquid has evaporated, add the last of the mushrooms and the final pinch of salt. Cook until browned, and liquid has evaporated.
Allow the mushrooms to cook until some of the bits are sticking to the bottom of the pan.
Add marsala wine, scraping the bottom to loosen the bits. Add stock and bring to a boil. Reduce heat to medium low and simmer for 20 minutes. Remove bundle of thyme.
If canning - fill sterilized pint jars, leaving 1 inch at the top. Wipe the edge clean using cider vinegar and a clean paper towel. Place lids on and fasten. If using a pressure canner follow the instructions for 10 pounds of pressure for 45 minutes. Let canner come to atmospheric pressure naturally, then open lid and transfer to a towel-lined rack to wool for 24 hours. Store in cool dry place for 1 year.
If freezing - fill air tight plastic containers with 1 pint of mushroom soup concentrate. Close, and store in freezer for 4-6 months.
To make Creamy Mushroom Soup -
Melt butter in a sauce pan over medium heat. Whisk in flour. Keep whisking and cook until it bubbles and starts to thin. Reduce heat to low and cook for 3-4 minutes.
Slowly add 1 pint of mushroom soup concentrate, whisking until incorporated. Add water, and bring to a boil. Reduce heat and simmer for 5 minutes. Whisk in heavy cream, do not boil.