Bake your crust, and allow to cool (if not already prepared)
Set a large saucepan with the 2 cups lactose-free half-and-half (or see below for alternatives) over medium heat
Bring cream to a boil, stirring occasionally
Note: you will be working concurrently with the egg mixture while the cream is slowly warming up. If the cream boils, then turn heat to low while prepping the eggs and sugar
While cream is warming, beat egg yolks in large bowl of electric mixer fitted with whisk attachment at medium-high speed until foamy
Gradually add sugar a bit at a time and continue beating until the mixture is pale yellow and thickened, scraping bowl as necessary
Sprinkle in corn starch and salt with beaters on low, then turn up to medium and whisk until blended
Return mixer back to low speed and if the cream is ready, slowly add it to the eggs. Pour, or spoon this into the egg mixture, a little at a time, to temper the eggs so they do not cook
Scrape sides and bottom of bowl, then increase to medium speed until well blended
Pour the egg/cream mixture back into the saucepan, and cook it over a medium-high heat, stirring constantly with a whisk, until boiling and thickened
Adjust heat as necessary to not burn the mixture (depending on your stove)
It will begin to thicken suddenly, watch your mixture to reduce heat at that time
Lower heat and cook for 1 minute more, continuing to stir so no lumps form
Remove from heat and stir in lemon juice, lemon zest, butter, and lemon extract
Allow to sit for about 10 minutes, then pour into your cooled, prepared crust and refrigerate until set; about 3-4 hours, or overnight
You will use about 2 lemons for the recipe, more if you use them for garnish. You will want to zest the lemons before juicing them. Please ensure you do not get the white pith in the mix as that is bitter tasting!
Although this recipe has not been lab tested, a single serving should be low-FODMAP based on the ingredients (using available FODMAP data at time of posting)
If you want a dairy-free option, I suggest full-fat canned coconut milk for the base and canned coconut cream for the topping (1/4 cup is one low-FODMAP serve), as well as a dairy-free butter alternative
Heavy whipping cream is low-FODMAP in 1/4 cup servings; butter is naturally low FODMAP due to low lactose levels