Place mushrooms on a flat dish and cover with tamari for at least 3 hours, turning occasionally. Dehydrate mushrooms in a slow oven set to 80°C until they are dry enough to mill.
Place mushrooms into Thermomix bowl and mill 6 sec/speed 6. Remove from bowl and set aside.
Place tomatoes into Thermomix bowl and cook 10 min/100°C/Reverse/speed 1.
Press cooled pulp through a fine sieve and separate solids from liquids. Reserve liquid to use in Umami paste.
Place Parmesan and walnuts into clean, dry Thermomix bowl and mill 8 sec/speed 9.
Add remaining ingredients including dried mushrooms and blend 30 sec/speed 4, scraping down as necessary.
Store in sterilised airtight jar in fridge and use within 3 weeks, or freeze in ice cube trays, then add a cube or two to dishes as needed.
To make a dairy-free Umami Paste, use this method with mushrooms in place of the parmesan.
To make a nut free Umami paste, replace the walnuts with toasted sunflower seeds.