This recipe makes apple butter that is less sweet than most; use more sugar if you like, but don’t use less or there won’t be enough to properly preserve the pureed apples. Make your first apple butter plain so you can taste the flavor of the apples. But if you prefer flavorings, add half a vanilla bean, a cinnamon stick or some grated nutmeg to the apple butter while it bakes.
Clean the apples, then remove the stems. Cut the apples into quarters — including the cores — then place into a large saucepan. Cover with 1 inch of cold water, then place the pot over high heat and bring to a boil. Once it comes to a full, rolling boil, reduce the heat to maintain a steady simmer and cook until the apples are very soft and a paring knife inserted through the pieces slides in and out easily, 20 to 30 minutes, depending on the variety. Remove the pot from the heat and let the apples cool in the water for 20 minutes.
Place a food mill over a large bowl. Using a slotted spoon and working in batches, transfer the apple pieces to the food mill and process the apples so the smooth purée falls into the bowl; clean out the food mill, discarding the skins and seeds after each batch to keep the process going smoothly. Once all the pieces are out of the cooking water, pour the water through a coarse strainer or colander and into a separate bowl to remove any remaining seeds or large pieces. Finally, pour the cooking water through a fine sieve to capture any leftover pulp; discard the water after this. Add the pulp in the strainer to the processed apple purée. You should have 1248 grams or around 5 1/4 cups of puréed apples.
Heat the oven to 350 degrees. Add the sugar, lemon juice and a large pinch of salt to the apple purée, and stir to combine. Transfer the mixture into a 9-by-13-inch glass or ceramic baking dish.
Place the dish in the oven and set a timer for 30 minutes. Once the timer goes off, open the oven and use a long-handled wooden spoon or silicone spatula to stir the mixture, making sure to scrape all four sides of the dish where the mixture is concentrating, as well as all along the bottom to ensure any parts of the mixture touching the dish get mixed into the middle sections.
Repeat this process until the mixture reduces to a thick purée, about 2 1/2 to 3 hours total. You’ll know the apple butter is done when you stir it and the purée no longer separates from the thin, watery liquid. Also, there is no need to skim any foam that rises to the surface of the apple butter since it will mix in and become one with the apple butter by the time it is finished cooking.
Remove the dish from the oven and, if storing in jars, use a ladle to transfer the hot apple butter into clean glass jars. Cover with lids, let cool to room temperature, then store the apple butter in the refrigerator. If storing the apple butter in plastic storage containers, let it cool to room temperature in the dish before transferring to the containers. Store the apple butter in the refrigerator for up to 3 months.