1. Zest one lemon and set aside. Slice the remaining 3 lemons in half lengthwise through the stem. Use a small paring knife to cut along the edge, then use a spoon to carefully scoop the flesh and membranes into a bowl. Strain and reserve 2 Tbsp + 1 tsp lemon juice. Set the hollowed lemon halves aside.
2. Add the heavy cream and sugar to a small saucepan over medium heat. Bring to a gentle boil, stirring occasionally, then simmer for 3 min.
3. Remove from the heat and stir in the 2 Tbsp + 1 tsp reserved lemon juice, lemon zest (saving a little for serving), and salt. Let sit for 5 min.
4. Arrange the hollowed lemon halves cut side up on a small sheet pan. If needed, place them in small bowls or use crumpled foil to help stabilize them. Transfer the custard into a glass measuring cup with a spout and carefully pour the mixture into the lemon halves, filling right to the rim. Transfer to the fridge and chill until set, at least 2 hours or overnight.
5. Right before serving, garnish with lemon zest, mint or thyme (if using), and flaky sea salt. Enjoy!
