Peel, core, and slice apples into ¼-inch thick slices for consistent cooking. Keep slices uniform thickness so they cook evenly. Use mix of Granny Smith and Honeycrisp - tart and sweet balance. Need about 6-7 large apples for one pie filling. Don't cut too thin or they turn to mush. Toss apple slices with lemon juice immediately to prevent browning.

Mix granulated sugar, brown sugar, ground cinnamon, nutmeg, and salt in bowl. Brown sugar adds molasses depth white sugar doesn't have. Cinnamon is main flavor but don't overdo it. Nutmeg adds warmth in background. This spice blend makes classic apple pie filling taste right.

Melt butter in large skillet or pot over medium heat. Add apple slices and toss to coat. Sprinkle sugar-spice mixture over apples. Add ¼ cup water to help dissolve sugar. Cook 8-10 minutes stirring occasionally until apples start to soften. They should be tender but still hold shape, not mushy.

Mix cornstarch with ¼ cup cold water to make slurry - this prevents lumps. Pour slurry into cooked apples while stirring constantly. Cook another 2-3 minutes until sauce thickens noticeably. The filling should coat back of spoon. This is crucial for thick filling that doesn't run. If too thick add splash of water, too thin add more cornstarch slurry.

Remove apple pie filling from heat. Let cool to room temperature before putting in pie crust - hot filling makes crust soggy. Spread on baking sheet to cool faster if in a hurry. Once cooled, ready for your pie. Can also freeze or can it for later.