Using a steamer basket, cook sunchokes through until easily pierced with a pairing knife (45 minutes-1 hour). Peel the sunchokes and cover, reserving skins for garnish. In a heavy-bottom pot, combine butter, thyme, shallot and porcini powder. Stir over medium high heat until butter begins to brown and shallots have caramelized. Add the sunchokes and continue stirring to break up any large pieces. Deglaze the pan with Madeira and lemon juice. Continue to stir for about two minutes. Add chicken stock, bring to a simmer and remove from heat. Allow the mixture to cool slightly and then carefully purée in a blender or food processor until smooth. While the motor is running, slowly incorporate the walnut oil. Chill the purée in the refrigerator, season to taste and reserve. Purée may be prepared up to two days prior to serving.
To make the vinaigrette, film a heavy skillet with two tablespoons of the olive oil over medium-high heat. Sear the lemons cut side down until nearly black. Turn lemons over, cook for one minute and remove to cool. To the same skillet, add fennel, shallot and garlic and stir until brown. Continue to stir and add spices while scraping the bottom of the pan carefully for 30 seconds. Remove the pan from heat and add the vinegar, honey and the juice of the grilled lemons. Pour the contents of the pan into a blender, liquefy and strain into a non-reactive bowl. Slowly whisk in oils to emulsify. Season and reserve in the refrigerator for up to two weeks.
Prepare a grill to medium high heat. Season the tenderloin with 1 tablespoon of olive oil, salt and pepper. Grill the pork, turning 4-5 times, to an internal temperature of 145 degrees (12-15 minutes); remove from heat and reserve warm. While pork is resting, toss onions in 1 tablespoon of olive oil, salt and pepper. Grill onions until soft throughout and on the edges (3-4 minutes). Remove from the grill and reserve warm.
Film a heavy pan with 2 tablespoons of olive oil over medium high heat. Julienne kale and sauté briefly, until just wilted. Steam through with a tablespoon of plancha vinaigrette (15-30 seconds).
To plate, divide warmed purée between six small plates. Top with kale, onions and tenderloin sliced across the grain. Season with plancha vinaigrette, fennel pollen, piment d’Espelette and sel gris. Garnish with fried sunchoke skins, if desired.