For the puree, boil cauliflower in salted water until very tender. Drain well, stir in heavy cream, and puree with an immersion blender. Add cubed butter and further blend to smooth out the puree. Season, to taste, with salt and white pepper and keep warm.
For the sauce, simmer beer, shallot, 1 tablespoon of the hops (reserving the rest), thyme, and white pepper until the liquid is reduced by half. Strain and allow to cool to room temperature.
Add the egg yolks to a double boiler fitted over a pan of barely simmering water. (In order to avoid making scrambled eggs, the water level should not touch the bottom of the double boiler bowl.) Begin beating the egg yolks with a whip and add 1 tablespoon of the beer reduction. Beat until the yolks are thick, about 5 minutes. Crumble the remaining hops into the reduction to intensify the flavor.
Remove the entire double boiler assembly (pot and all) to a heatproof surface like a large cutting board. Lift the double boiler from the pot and place a clean damp kitchen towel over the pot, tuck the loose ends of the towel under the base of the pot, and return the double boiler bowl to its position so that it sits atop the damp towel. Pour some of the clarified butter into the bowl in a slow steady stream while whisking constantly. As the mixture thickens, alternate some of the beer reduction with the butter, until all of the butter and the reduction are emulsified. Add the cayenne pepper and lemon juice. Add salt, if needed. Adjust the thickness of the sauce by beating in additional warm water. Set aside in a warm area until ready to use. (Please note that this sauce cannot be made in advance, so try to time it as closely as possible to serving time.)
Heat the grapeseed oil in a large skillet over medium-high heat until it shimmers. Season the salmon fillets with salt and pepper and sear flesh-side down first, leaving undisturbed for the first few minutes to allow the caramelization process to begin and to prevent tearing of the flesh. To complement the taste of the beer reduction, it would be desirable to put a hard sear on the salmon, which means leaving it to sear until it is a dark (but not burnt) golden brown. Flip the fish and sear the other side the same way. Remove from heat to let rest and cover to carryover cook until it flakes.
Spoon some cauliflower puree on plate. Top with a piece of salmon, some sauce, and a sprig of chervil.