From America's Test Kitchen
In the test kitchen, we have found that it’s difficult to produce an apple pie with a filling that is tart as well as sweet and juicy. We wanted to develop a classic apple pie recipe—one with the clean, bright taste of apples that could be made year-round, based on apple types that are always available in the supermarket.
To arrive at the tartness and texture we were after, we had to use two kinds of apples in our pie, Granny Smith and McIntosh. The Grannies could be counted on for tartness and for keeping their shape during cooking; the Macs added flavor, and their otherwise frustrating tendency to become mushy was a virtue, providing a nice, juicy base for the harder Grannies. While many bakers add butter to their apple pie fillings, we found that it dulled the fresh taste of the apples and so did without it. Lemon juice, however, was essential, counterbalancing the sweetness of the apples.
If you are making this pie during the fall apple season, when many local varieties may be available, follow the recipe below using Macoun, Royal Gala, Empire, Winesap, Rhode Island Greening or Cortland apples. These are well-balanced apples, unlike Granny Smith, and work well on their own without thickeners or the addition of McIntosh. Placing the pie on a baking sheet in the oven inhibits cooking, so cover the bottom of the oven with a sheet of aluminum foil to catch a dripping juices. The pie is best eaten when cooled almost to room temperature, or even the next day.