Place porcinis in a bowl that’s large enough to contain the 4 cups vegetable broth. If the porcinis look large, tear them into bite sized pieces. Boil the vegetable broth and pour it over the porcinis. Cover with a plate to keep hot, and prep everything you need for the recipes. This will soften them and make the recipe a bit faster.
Preheat a stovetop-to-oven dutch oven over medium heat. (See notes for non-dutch oven directions.) Saute onions and a pinch of salt in oil until until translucent, 4 to 7 minutes. Add garlic, for about a minute, until fragrant.
Add sliced cremini mushroom (not the porcinis yet), celery, thyme and rosemary (crush herbs in your fingers) and saute for 5 minutes, until mushrooms release moisture and brown slightly.
Add carrots, stout, tomato paste, fresh black pepper and salt and bring to a boil. The liquid should reduce in about 3 minutes.
Add porcinis and vegetable broth, cover and bring to a full boil for 5 minutes or so, to finish cooking the porcinis.
Once porcinis are soft, in a measuring cup, mix the flour into one cup of cold water with a fork until no lumps are left.
Slowly add the water/flour to the pot, mixing well as you go. Let thicken for 5 minutes or so. Add the kidney beans, turn heat off and cover to keep warm. In the meantime, preheat the oven to 425F and make the biscuits.