Roasted King Crab Legs with Jalapeño-Hazelnut Vinaigrette and Black Trumpet and Grapefruit Crumble

This is a fun dish of opposites—sweet, tender crab with savory, chewy mushrooms, the hot crumble and the cold crab, and the real star, besides the crab itself, the jalapeño vinaigrette, which is a beautiful green and deeply flavored with the jalapeño and hazelnut oil. Black trumpet mushrooms should always be blanched in water to which a generous handful of sugar has been added (in the same way you would salt water for pasta) to counteract the residual bitterness they often have. The crumble