Soak the gelatin sheets in cold water for 10 to 15 minutes. If using powdered gelatin, rehydrate it in 6 times its weight in water and let it swell in the refrigerator.
Whiten the egg yolks and sugar together to dissolve the sugar.
Add the cornstarch and the zest of one lemon. Mix well.
Bring the milk to a boil.
Pour it over the blanched mixture in two additions while whisking to mix and obtain a homogeneous preparation.
Transfer everything back to the saucepan.
Heat over low heat to thicken the custard. Stir continuously in a figure-eight motion with a spatula until the custard coats the spatula well.
If you run your finger over it, it should leave a clear line. If you have a thermometer, remove the custard at 83-84°C (181-183°F). Be careful not to let the custard boil.
If you are afraid the custard will curdle and become grainy, remove it from the heat several times while stirring constantly or cook it in a double boiler.
Off the heat, add the drained gelatin and mix well. Add the lemon juice, mix, and then strain through a sieve to remove the lemon zest and any residue.
Cool quickly, stirring occasionally. You can speed up cooling by placing the bowl on a container with ice cubes.
Let cool to 30°C (86°F) before incorporating the whipped cream. If you don't have a thermometer, it's fine; just let the custard cool well without letting it set. It should still be liquid to mix easily with the cream.
Whip the cold cream until quite firm but not too stiff.
Incorporate the custard into the whipped cream in three additions, mixing with a spatula from bottom to top. In many recipes, you can see the whipped cream added to the custard. (See note below)
Take the ring with the sponge and praline crisp and pour the lemon bavarois into it.
Gently shake the ring to smooth the mousse.
Let it set a bit on the work surface, then place it in the freezer until the next day. The next day, prepare the Italian meringue.