Put the1/2 cup dried porcini mushrooms in a small bowl and cover with 3 cups of hot water. Let soak for 30 minutes.
Whisk the 2 tbsp flour with the 1/4 cup water until smooth and creamy. Set aside.
Heat the 2 tbsp olive oil (or 3 tbsp water or vegetable broth) in a medium skillet over medium-high heat. When the oil is hot, add the onions and mushrooms and cook for 7 minutes ,or until they begin to brown. Add the shallots and cook for two minutes. Add the garlic and cook for one to two minutes, or until it just begins to brown..
Pour the porcini mushrooms, including the water they soaked in, into the pan. Add the 1 3/4 cups of no-chicken broth and the rosemary and thyme and bring to a boil. Reduce heat to simmer and cook for 15 minutes.. It's normal for the liquid to reduce during this time, but you don't want it to evaporate too much, so make sure that the heat is low.
Put a fine strainer over a large bowl and pour the mushroom mixture into the strainer. Rinse the pan and put back on medium-high heat. Pour the mushroom gravy back into the pan and bring to a low boil. Whisk in the flour mixture and continue whisking until the gravy thickens Add the salt and pepper to taste and stir for another minute.
Strain the gravy one more time if needed to make sure that it’s smooth and creamy.
Don’t throw the solid mushroom mixture away. Remove the herbs and finally chop it, then serve as a side dish, or save it to toss with pasta or quinoa.