Step 1Combine 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 2Add 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.