Sweat the shallots in a covered 1-quart saucepan with 2 tablespoons of water until they are soft, about 5 minutes. Add the Madeira and the thyme and simmer, uncovered, until reduced by half. Add the meat jus and cook for another 2 minutes. Strain, discarding the shallots and thyme.
Return to the same pan and boil until the sauce is reduced by one quarter and has a nice consistency. Add the truffle and cook for 1 minute. Whisk in the butter and lemon juice. Season with salt.