Preheat oven to 100˚C. For salmoriglio sauce, place oil and lemongrass in a small ovenproof saucepan and heat to 100˚C. Place in oven and slow-cook to infuse (1 hour). Remove from oven and cool completely. Discard lemongrass. Place all ingredients in a bowl, season to taste and stir to combine. Set aside.
Increase oven to 200˚C fan-forced. Meanwhile, for tomato caramel, place tomatoes in a small, flameproof roasting pan, scatter with sugar and drizzle with a little oil. Roast until caramelised and tomatoes are sticking to base of pan (30-35 minutes). Remove from oven and place pan over medium-high heat. Add 250ml water and using a wooden spoon, stir tomatoes scraping base of pan to release the caramel. Cook, stirring frequently, until a thickened caramel sauce begins to form (6-8 minutes). Transfer to a blender, taking care of the heat, blend until smooth and combined. Set aside and keep warm until ready to serve.
Meanwhile, preheat a lightly greased barbecue or char-grill pan to high. Place collars in a large bowl, season to taste and drizzle with oil. Toss gently to combine. Cook collars, turning frequently, until charred and cooked (5-6 minutes). Rest for 5 minutes before serving.
Serve collar with salmoriglio, tomato caramel and a sprinkling of piment d'espelette.
