When ready to bake pie, adjust the oven racks so one is in the lower position and another is in the middle of the oven. Preheat oven to 400 degrees. Line a baking sheet with parchment paper.

Combine flour, oats, sugar, salt and cinnamon in a food processor. Pulse several times to mix.

Scatter the butter pieces over the dry mixture. Pulse until the mixture resembles coarse crumbs. Don’t over do it, you want to see some oat flakes.

Empty the crumbs into a large bol and rub them gently to even out the consistency and make them more ‘buttery’. Store in fridge until after layering apples in next stage.

Mix the sugar, cornstarch and salt together in a small bowl. Combine the apples and lemon juice in a large bowl. Sprinkle about 1 tablespoon of sugar mixture evenly over the pie shell. Arrange a single compact layer of apples in the shell, flat side down. Sprinkle with 1 tablespoon of sugar mixture. Arrange a second layer of apples on top of the first, and sprinkle with another tablespoon of the sugar mixture. Continue layering until all the apples and sugar mixture have been used.

Set the pie on the prepared baking sheet and cover with a generous amount of the crumb topping. You may not need all of it. Tamp the crumbs down lightly.

Bake the pie on the sheet on the lower oven rack for 30 minutes @ 400 degrees. Reduce the heat to 375 degrees and move the pie up to the middle rack, rotating it 180 degrees. Bake for up to 30-40 minutes longer, until you see thick juices bubbling up around the pie. If the top starts to get too dark, cover with foil.

Transfer the pie to a rack and cool or at lest 1 ½ hours to 2 hours before serving. Cover and refrigerate left-overs after 24 hours.