Put the mushrooms in a medium heatproof bowl. Pour in 2 cups boiling water and weight down the mushrooms with a small plate so they are submerged. Soak until they're plumped and softened, about 20 min. Use a slotted spoon to transfer the mushrooms to a cutting board, squeezing any excess liquid from the mushrooms back into the soaking liquid. Let cool. Remove and discard any tough stems. Coarsely chop the mushrooms. Strain the soaking liquid through a coffee filter or paper towel set in a sieve and reserve.
Heat the butter in a 10- or 12-inch nonstick skillet over medium heat until it melts and begins to foam. Add the shallots, sprinkle with a pinch of salt, and cook, stirring, until they soften and become translucent, 2 to 3 min. Add the mushrooms and cook, stirring occasionally, until they start to brown in places, about 5 min. Add the Cognac or brandy and cook, stirring, for 1 minute. Add all of the mushroom soaking liquid, the soy sauce, and thyme and cook until the liquid reduces by about half, 5 to 7 min.
If using right away, stir in the chives and cream (or butter). Season to taste with salt, pepper, and lemon juice, if using.
Stored in an airtight container in the refrigerator, the ragoût will keep for four to five days. Reheat over low heat, adding a couple of tablespoons of water, if necessary, to keep the sauce moist. Add the chives and cream and season to taste with salt and pepper and lemon juice, if using.