Use a peeler or microplane to zest your lemons, being careful to avoid the white pith. You're aiming for thin strips of the yellow peel only.
25 g lemon zest
Place the lemon zest into a large jar or multiple medium-sized mason jars that can be submerged in your water bath. Pour in the high-proof alcohol. Screw the jar lids on finger-tight—just enough to keep water out.
750 ml Everclear 190-proof
Preheat your sous vide bath to 130°F (54°C). Submerge your sealed jars and infuse for 2 hours. The alcohol must be completely below water level, even if the jar itself is too tall for the water bath.
While the infusion is going, combine the water and sugar in a saucepan over medium heat. Stir until sugar dissolves completely, then cool to room temperature.
17 ounces filtered or distilled water, 2½ cups granulated sugar
Remove the jar from the bath and let it cool slightly. Strain the lemon zest from the alcohol using a fine mesh sieve. If you used the microplane and have tiny bits of zest, use a cheesecloth in the sieve to strain them out.
Slowly stir in the cooled simple syrup, tasting as you go. Add less or more syrup depending on how sweet and strong you like it. For a more traditional limoncello, use the full amount.
Pour into bottles using a funnel. Store in the fridge or freezer and let it rest at least 3 day to mellow and blend. The longer it rests, the smoother it gets.
Serve ice-cold in small glasses after dinner, swirl into a cocktail, or gift in pretty bottles with a bow.