Prepare a mesh strainer by setting it over a medium heatproof bowl. Set the strainer and bowl aside near your stove.
Fill a medium saucepan with 3-4 inches of water and bring it to a simmer over medium heat.
While the water is heating, combine the allulose, lemon juice, and lemon zest in a large, heatproof bowl and whisk to combine. Add the eggs and egg yolks and immediately whisk to combine.
Place the bowl with the lemon mixture over the simmering water to form a double boiler. Whisk the lemon mixture constantly. The allulose will dissolve and then the mixture will begin to thicken as the eggs cook. The lemon mixture is done cooking when it's reached 160° F. You'll know it's ready when it's reached the consistency of sour cream or full fat yogurt.
Turn off the burner and remove the bowl from the double boiler, being careful not to burn yourself on the escaping steam. Immediately strain the hot lemon mixture through the prepared mesh strainer and into the bowl that you previously prepared next to your stove. Tap the strainer firmly and use a rubber spatula to help the curd through without damaging your strainer.
Add the butter, vanilla, and salt to the strained lemon mixture while it's still hot. Whisk the mixture occasionally, until the butter has melted completely and is fully incorporated.
Pour the finished lemon filling into the prepared pie crust and then set aside while you make the meringue topping.