In a medium saucepan, combine mushrooms, shallot, garlic, thyme, lemon juice, salt and pepper. Add enough water to just top mushrooms. Cover and bring to a simmer over medium heat. Cook 4 to 5 minutes, or until mushrooms are tender.

Remove pan from heat and let stand until mushrooms are cool. Strain mushrooms in a sieve set over a medium bowl, reserving the liquid. Let drain well.

Place mushrooms in a blender and, with motor running, add olive oil in a thin, steady stream, blending until mushrooms are pureed. Add truffle oil and blend until smooth. If puree is too thick, add a little of the reserved liquid; taste and adjust seasoning if necessary. With a rubber spatula, turn puree into a small serving bowl. The puree can be covered and refrigerated up to 1 day. Bring to room temperature before serving with toasted baguette slices.