Marital bliss was short-lived, and Reynolds complained bitterly to Cyril about Alma, who had overheard. It’s time to get Reynolds in line again, and the next time they’re out at the cottage, she cooks him an omelette with the same poisonous mushrooms she used to give him before.
Throughout the scene, we get callbacks to what usually upsets Reynolds as Alma asserts her authority over him and he feels comfortable letting her go. Before that, we found out that he just likes mushrooms cooked with a little butter; she scooped the above. During breakfast, we found him annoyed with her for pouring the tea so loudly; now she pours him a glass of water by dripping water into the cup from a great height.
Reynolds realizes the omelette is poisoned, but he eats it anyway because Alma says she wants him weak and helpless just for her to take care of him, then make him strong again. Again, she adopts the tone of a parent, saying he needs “to settle down a bit.” Reynolds seemed almost amused by the prospect and kissed her, the first appropriate moment of romantic passion we saw from him.
There’s obviously an element of sadness in this exchange of dominant roles, but an important one for the survival of their relationship. Reynolds understands that poison helps eliminate his toxic behavior and become a loving husband, and Alma has no qualms about pushing him to get there in his character.
It’s a dangerous game that could end up killing Reynolds, but while dragging, it gives them a happy ending. Alma dreams of their future, and we are left with an image of them wearing a dress together and facing each other as usual.
https://www.slashfilm.com/1037814/phantom-thread-ending-explained-dressing-down-the-dressmaker/ Phantom Thread: Dressing Down The Dressmaker Ending Explained