Step 1 Combine 4 cups water, one-fourth cup of the honey, the sugar, lemon slice and peppercorns in a large saucepan. Using a knife, scrape the seeds from the vanilla bean and add to the mixture. Bring to a boil. Lower the heat and simmer 10 minutes to blend the flavors.
Step 2 Add the pears and simmer until they are tender, about 15 to 20 minutes. Remove the pan from the heat and lift the pears into a glass bowl, then pour over the juices. Let cool to warm. Refrigerate at least 1 hour, or up to overnight.
Step 3When you are ready to serve, remove the peel and stem from the pears. Halve each pear lengthwise and remove any seeds, if necessary. Cut each pear half into 4 wedges.
Step 4Arrange two pear wedges on a plate and squeeze a few drops of lemon juice over them. Spoon 2 tablespoons of the yogurt on top. Drizzle 1 teaspoon of the honey over the yogurt and around the pears on the plate. Repeat with the three remaining plates. Toss the toasted walnuts with the melted butter and a light sprinkling of fleur de sel, then scatter the walnuts evenly over the plates. Serve immediately.