Step 1Beat together the butter, shallot, chervil, chives and tarragon until the mixture is relatively smooth and pale green. Beat in the lemon juice. Taste and season with one-fourth teaspoon salt. The butter should taste aggressively lemony.
Step 2Spoon the butter onto a sheet of plastic wrap in the shape of a log and roll it into a cylinder. Twist the ends of the plastic wrap in opposite directions to firm the cylinder and roll it gently on the counter to eliminate any air pockets in the center. Refrigerate until ready to use. (Recipe can be prepared to this point a couple of days in advance).
Step 3To prepare the packets, place a sheet of aluminum foil on the work counter. Arrange 1 cup of pea sprouts in a bed in the center. Lightly salt a halibut steak or fillet on both sides and place it on the pea sprouts. Slice off a generous tablespoon of the herb butter and place it on top of the halibut. Seal the packet tightly, folding the edges over several times to ensure a tight seal. (Recipe can be prepared to this point 8 hours in advance and refrigerated until ready to bake.)
Step 4When ready to bake, heat the oven to 400 degrees. Place the sealed packets on a jellyroll pan and bake 12 to 15 minutes (the longer period if the packets have just come from the refrigerator, or if you like your fish more thoroughly cooked).
Step 5Transfer the baked packets to a plate and serve immediately, warning your guests that they are full of very hot, very fragrant, steam.