Par-cook the lobster: Bring a large pot of water to a rolling boil with the lemon juice, 1 tsp of the salt and the thyme sprigs. Drop in the lobster tails and cook for exactly 4 minutes, until the shells turn bright orange. Drain, cool slightly, then carefully work the meat out of the shells in one piece. Pat completely dry and set aside.
Dry-roast the duxelle: Put the finely chopped mushrooms in a dry skillet over high heat with the thyme leaves, a pinch of salt and a few grinds of pepper. Cook, stirring often, for 8 to 10 minutes until all the moisture has cooked off and you have a dark, dry paste. Spread it on a plate and let it cool completely.
Wilt the spinach: Melt the butter in the same skillet over medium heat. Add the garlic and cook for 30 seconds, then pile in the spinach and toss for about 1 minute until just wilted. Transfer to a sieve, drain well, then squeeze out every last drop of water with your hands. Chop, mix with the lemon zest, and season with salt and pepper.
Make the herbed crepe: Whisk the flour, egg, milk, chives and salt until completely smooth. Ladle a thin layer into a hot nonstick pan, swirling to coat, and cook for 1 to 2 minutes per side until golden. You need one large crepe — leave it to cool.
Roll the log: Lay a large sheet of plastic wrap on the work surface and place the crepe flat on top. Spread the spinach in an even layer over the crepe, then spread the cooled duxelle over the spinach. Lay the four lobster tails end to end in a line down the center and season with the remaining 1/2 tsp salt and pepper. Use the plastic wrap to roll everything into a tight, neat log, twisting the ends like a cracker. Chill for 20 minutes.
Wrap in pastry: Roll the puff pastry out on a floured surface to about 1/8 inch thick. Unwrap the lobster log, place it seam-side down on the pastry, and brush the pastry edges with beaten egg yolk. Fold the pastry around the log, press to seal, and trim the excess. Turn seam-side down onto a parchment-lined baking sheet. Brush all over with egg wash and chill for 15 minutes — cold pastry bakes flakier.
Bake golden and rest: Heat the oven to 425°F. Score the pastry lightly in a crisscross or lattice pattern with the back of a knife (do not cut through), then egg wash once more. Bake for 20 to 25 minutes until deeply golden and puffed. Rest for 5 minutes before slicing into thick portions with a sharp serrated knife.