Step 1In a saucepan over medium heat, warm the milk to 105 to 110 degrees. Pour it into a large bowl, add the yeast and stir to dissolve. When the yeast gets slightly foamy (after about 10 minutes), stir in one-half cup of flour. Cover with plastic wrap; set aside for an hour in a warm place.
Step 2Heat the oven to 350 degrees. Place four eggs, the yeast mixture, 2 1/4 cups of the flour, and the salt and sugar in the bowl of a 5-quart stand mixer. With the paddle attachment, beat the mixture on medium speed until well mixed. Continuing on medium speed, slowly add in the butter and truffle butter.
Step 3When completely mixed, the dough should be fairly wet but pull away cleanly from the sides of the bowl. Add flour as necessary, up to 1 cup. When the mixture starts to pull together, switch to a dough hook and knead for a few minutes.
Step 4Remove the dough from the mixer and shape into six flattened balls about half the size of the buns that you want. Place the balls on a greased and floured cookie sheet, leaving space for them to rise. Cover with a damp towel or plastic wrap sprayed with cooking spray. Place the buns in a warm place, and allow them to rise until doubled in size, about 1 hour.
Step 5Brush the buns with an egg wash -- one egg beaten with 1 tablespoon of water. Bake for about 25 minutes, or until the tops are a deep golden brown and a skewer inserted into the middle comes out clean. Cool on a wire rack before cutting.
Step 1In a small saucepan, bring the vinegar, 1 cup of water, salt and peppercorns to a boil.
Step 2Cut the onion into one-half-inch thick slices. In a heatproof bowl, pour the boiling liquid over the onion.
Step 3Cool the mixture to room temperature, and then store in the refrigerator for at least a couple of hours and ideally overnight.
Step 1Heat the oven to 250 degrees.
Step 2Cut the tomatoes into half-inch-thick slices and season both sides with salt and pepper. Lay the slices on a metal cooling rack set over a sheet tray and brush them with olive oil.
Step 3Bake the tomatoes for about 2 hours, or until they shrink by half and are about half-dried. Cool and reserve.
Step 1In a small food processor, combine the egg yolks, salt, lemon juice and truffle pieces (if using).
Step 2With the food processor on, slowly add the canola oil in a thin stream into the eggs. Finish by adding the truffle oil. Adjust the seasoning to taste. Cool and reserve.
Step 1In a small saucepan over medium heat, bring 1 cup of water, the tomato paste, sugar, vinegar and salt to a simmer. Cook, stirring frequently, until a ketchup-like consistency has been reached, about 10 minutes. Adjust the seasoning to taste. Cool and reserve.
Step 1Grill the hamburgers over medium-high heat, preferably over wood charcoal (ideally oak) until medium rare, 3 to 5 minutes per side.
Step 2Cover the burgers with slices of the truffle cheese and allow the cheese to melt.
Step 3Brush the cut sides of the buns with truffle butter and grill, cut side down, until slightly toasted.
Step 4Place the burgers on the buns and garnish with the watercress, oven-dried tomato, pickled onions, optional shaved truffle, truffle ketchup and truffle aioli. Serve immediately.