Heat medium to large size flat-bottom skillet to medium heat. Add butter; melt. Add shallots, sautee for 30 seconds. Add mushrooms and sautee until brown on both sides. Meanwhile, slice 2-4 slices of lemon for garnish; keep the remaining 2/3 of lemon whole (for juice).

Add wine to deglaze the pan (scrape any carmelized stuff from the pan into the wine), and add the fresh juice from the 2/3rds of the lemon. Bring heat back to medium high and reduce, stirring once or twice, until the wine evaporates and the mixture carmelized (2-4 minutes).

Remove carmelized mushrooms and shallots from heat and add cream and chopped tarragon. Stir, and place back on low (or remove from heat altogether until ready to serve). Serve warm (ideally, immediately).