Combine mushrooms and boiling water in a bowl; cover and let stand 15 minutes. Drain mushrooms in a colander over a bowl, reserving 1/4 cup liquid. Rinse and drain mushrooms; finely chop.
Melt butter in a small saucepan over medium heat. Add shallots; cook 3 minutes or until tender, stirring frequently. Add sherry; cook 1 minute. Add mushrooms, reserved mushroom liquid, stock, and thyme; bring to a boil. Reduce heat; simmer 10 minutes. Gradually add flour, stirring with a whisk. Bring to a boil; cook 1 minute or until slightly thick, stirring constantly with a whisk. Stir in salt and pepper.