The flavors in this soup merge to create a unified and rich, yet surprisingly light dish. By slow-cooking the onions, flavors develop that resemble French onion soup, yet the potatoes give it a creamy texture, without the addition of any butter or roux. The ingredient list specifies two different varieties of bolete. You can simply use a whole cup of dried porcini (Boletus edulis) if you wish, but the flavor might get very intense. By stretching the porcini with lower grade species that have a similar texture, you can add complexity to the strong bolete flavor, while giving you a chance to use up any collections of lesser mushrooms.
Pre-soak the dried mushrooms in 1 cup of water for about 30 minutes. Boil or microwave the potatoes until soft (about 20 minutes.) When they are done, set them aside to cool slightly.
Chop the onions and put them in a large soup pot with the mushrooms and their liquid. Bring the liquid to a boil and stir occasionally. As the liquid evaporates, add an additional cup of broth. Continue reducing the liquid for about 30 minutes.
Chop the potatoes into large chunks, to make it easier to put them in a food processor. Combine the potatoes with the cooked onion/mushroom combination. Add the fresh tarragon and lovage sprigs, coarsely chopped. Add one cup of broth to increase the moisture content for blending. Place the combined ingredients in a food processor or blender (in quantities that it can handle), and thoroughly blend them together. Add more water for blending if needed.
Now place the ingredients back in the pot, and add one cup of beer and 2-3 cups of water. Bring back to a low boil, stirring for a few minutes, then serve. Makes enough for four large bowls.