Mixture will appear curdled in step 1 but will come together when cooked. Do not let it come to a boil. If it does overcook and curdles, pour curd through a fine-mesh sieve when done cooking to strain out the curdled parts.
If you like, this can also be made with lime, grapefruit, or your favorite citrus instead of lemon.
If you aren't planning on using it right away, transfer to clean jars or a tightly covered container. Covered tightly, it will keep in the refrigerator for about a week and in the freezer for up to 2 months.