French Defense Tarrasch Variation Puzzle (Rated 1226): Sac on a3 — Queenside

From the French Defense Tarrasch Variation comes a model queenside attack that opens with a queen sacrifice. White's queenside pawn shield is gone and the king is marooned near the corner, so Black strikes on the a-file with check, forcing the king onto a cramped square. White's pieces become tied down, and a knight leaps into an outpost on the queenside, hammering the queen while clearing the diagonal for a fianchettoed bishop. The attack nets Black decisive material.