Hand Method- In a large bowl, sift the flour and salt. Cut the chilled butter and margarine into 1-tablespoon bits and add to the flour. With a pastry cutter, work flour and shortening together until mixture resembles coarse meal. Add the ice water little by little pressing the pastry together into a ball. Wrap and chill for at least 1 hour.
It is very important to work the pastry as little as possible. Don’t over handle. A secret to light, flaky pastry is to keep the mixture cool, add as little water as possible, and mix only as much as necessary.
Food Processor Method- Put flour and salt in bowl of machine. Cut butter and margarine into flour. Process a few seconds until mixture resembles coarse meal. Drop by drop add the water, processing very briefly. The whole process would take 20 to 30 seconds. Divide the dough into two equal portions. Shape into a disc and wrap with plastic wrap. Chill the pastry for at least 1 hour.
While pastry is chilling, make apple filling and streusel topping.
Juice the lemons into a large mixing bowl. Core, peel, and thinly slice the whole apples.
Dredge all the apple slices in the freshly squeezed lemon juice to prevent browning and to add flavor. Toss the apples in the corn starch. Set the prepared apples aside.
In a medium-sized skillet on medium heat, toss in the brown sugar and butter. When the butter has melted, add in the berry jam. Toss the apples into the brown sugar mixture. Cook them in the mixture for about 5 minutes. Set aside to cool and prepare the topping.
Blend first 5 ingredients in processor. Add butter; using on/off turns, blend until moist dough forms (mixture will resemble wet sand). Add oats; using on/off turns, mix briefly, leaving half of oats whole. Set aside.
If pastry has been chilled for a long time, let it sit at room temperature for at least 15 minutes before rolling.
Preheat oven to 375 F.
Lightly flour a pastry board, marble counter, or kitchen counter. Pat one pastry disc (save other portion for another pie)into a flat round. Lightly flour the rolling pin. Roll pastry in one direction only, turning pastry continually to prevent it from sticking to the surface.
Using pie plate as a guide, measure rolled-out pastry — it should be slightly larger than the pie plate and 1/8-inch thick. Fold rolled pastry circle in half so you can lift it more easily. Unfold, gently fitting the pastry into the pie plate. There will be some of the pastry hanging over. You could trim a little around with scissors and leave enough to crimp.
Fill with your apple filling.
Sprinkle streusel topping evenly over filling.
Place pie on baking sheet.
Bake pie until crust is golden brown and juices bubble, 50 minutes–1 hour. *My pie was ready at 45 minutes. Every oven is different. I didn't want my topping to get too brown.
Transfer to a wire rack to cool.