1. In a small, heat-proof bowl, combine the dried porcini mushrooms and boiling water. Let stand for 20 minutes. Remove the mushrooms with a slotted spoon and chop. Set aside.
2. Line a fine-mesh sieve with a coffee filter or cheesecloth and strain the mushroom-soaking liquid. Reserve the liquid.
3. Cook linguine according to package directions and drain.
4. While pasta is cooking, heat the olive oil over medium heat in a large frying pan. Add the shallots and saute until soft and translucent (2-3 minutes). Add the garlic and saute for 1 minute. Add the shiitake mushrooms and saute until they brown around the edges (3-4 minutes).
5. Stir in the cremini mushrooms, porcini mushrooms, reserved soaking liquid, stock, and salt. Cover and cook until the mushrooms are tender (4-5 minutes).
6. Uncover and stir in the butter, chopped thyme, and pepper.
7. To serve, top portions of pasta with mushroom sauce and garnish with thyme.