First, in a small bowl, soak the dried mushrooms in hot water for 10-15 minutes, until soft. Then drain the mushrooms in a fine-mesh sieve, rinse with warm water to remove any remaining grit and squeeze out any excess water. Chop the mushrooms and set aside.
Meanwhile, peel and dice the onion.
Heat the oil in a medium saucepan over medium-high heat. Add the onion and cook, stirring occasionally, until browned, about 8-10 minutes.
While the onion cooks, tie up the thyme springs with twine. If using a bay leaf, you can add it to the thyme bundle or leave it separate. Set aside.
Add the tomato paste to the onion and cook, stirring frequently, until one shade darker in color, about 2-3 minutes.
Add 1 cup of the broth and use a spatula to scrape up the brown bits from the bottom of the pan.
Add the remaining 3 cups broth, the thyme springs (or dried thyme), bay leaf and chopped mushrooms. Turn the heat to high to bring it to a boil. Then reduce the heat so the gravy simmers for about 10-12 minutes, stirring occasionally.
Use the spatula or cooking tongs to remove the thyme bundle and bay leaf.
If using pan juices from chicken or turkey, reserve those for the time being. If not using pan juices, add 1 teaspoon fine sea salt to the gravy. Also add lots of freshly ground black pepper.
Turn off the heat and blend the gravy one of two ways:Blend directly in the saucepan with a stick blender - this is easier but will require more time to get it as smooth as possible.Blend in a regular blender - this is a bit more complicated but produces a beautifully smooth gravy. Add gravy to fill about halfway up the blender jar. Cover with the lid but remove the inner insert from the lid. Hold a kitchen towel tightly over the opening with one hand. Blend, starting on low speed and then turning up to medium, for at least 1 minute. If you have to blend the gravy in batches, pour out the blended gravy into a bowl and repeat the process with the remaining gravy. Then add all the gravy back to the saucepan. Place the blended gravy over medium heat.
In a small bowl, whisk or stir one of the following:1 tablespoon arrowroot flour/starch with 6 tablespoons gravy1 tablespoon coconut flour with 9 tablespoons gravy3 tablespoons cassava flour with 6 tablespoons gravy Whisk or stir the gravy in the saucepan as you slowly pour in the thickening mixture.
If you're using the pan juices from chicken or turkey, stir them in now.
Taste the gravy and adjust any seasonings. If the gravy isn't thick enough, let it simmer until it's reduced and thicker. If the gravy is too thick, add water or broth to reach the desired consistency. Serve hot.Recipe yields 4½ cups.