Stir together butter, shallot, zest, lime juice, chile, and salt in a bowl. Chill butter, but do not allow to harden.
TO MAKE BEDDING -
Bring 2 quarts of salted water to a boil. Blanch rapini for 90 seconds. Remove and plunge in ice bath. Remove from bath, drain and set aside, season with fresh ground black pepper and cover with damp paper towel.
TO MAKE FISH -
Pat fish dry and sprinkle with salt. Heat 1 tablespoon oil in a 12-inch nonstick skillet over moderately high heat until just smoking, then sauté 2 pieces of fish, turning over once with a spatula, until golden and just cooked through, 4 to 5 minutes, and transfer to a plate. Sauté remaining fish in remaining tablespoon oil in same manner.
Serve on bed of steamed rapini. Serve each piece of fish with a dollop of chile lime butter.