In a large bowl, toss the apples with the lemon juice. This prevents browning. Set aside.
In a large dutch oven or saucepan, stir together both the sugars, cornstarch cinnamon, nutmeg and salt. Add 4 cups of water and bring to a low simmer.
Add the apples and return to a low simmer. Cover and cook for 6-10 minutes. If you cut your apples into bite-sized pieces, it will be closer to 6 minutes, but larger slices will be 10 minutes. Apples should easily pierce with a fork and sauce will have reduced and thickened.
Remove from heat and allow to cool.
Use after cooling, can or freeze.
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Notes
This is enough filling for two standard 9-inch pies. Also feel free to omit cinnamon and nutmeg and include 1 1/2 teaspoons of Apple Pie Spice.