Preheat oven to 250°F.
Make the meringue layers: Put egg whites and vinegar into the bowl of a stand mixer. With the whisk attachment, whip egg whites until they begin to form stiff peaks (about 6-8 minutes). Once the egg whites are stiff, add the salt and gradually add the sugar until all the sugar is added and the whites are stiff and shiny. Put the meringue into a piping bag.
Line a sheet tray with parchment paper. Measure the bottom of your trifle dash so you know how large to make your meringue layers. With a pencil, trace three of the appropriate size disks onto the parchment paper. Pipe the meringue into the disks, about 1 inch thick, being very careful not to pipe outside the lines. If you have leftover meringue, pipe a few dollops for decoration.
Bake the meringues in the oven for 2 hours or until the meringues are dry on top. Turn the oven off and leave the meringues in the oven until completely cool. You can also leave them in overnight to ensure dryness.
Make the lemon poppy cream: In a stand mixer, whip the heavy cream until it forms soft peaks, about 3 minutes. Add powdered sugar and salt, and whip a little more just to incorporate. Remove the bowl from the mixer and fold the lemon curd into the whipped cream using a large rubber spatula. Add the poppy seeds.
In large trifle dish or glass bowl, build the trifle: (Tip: to make the layering easier, place the lemon curd into piping bags.) Place one of the meringue layers into the bottom of the dish and pipe the lemon cream on top. Repeat twice, ending with lemon cream on top.
Place the trifle in fridge for a few hours or overnight.
Just before serving, sprinkle with a little more poppy seeds and decorate with a few slices of lemon.