Place the raspberries in a colander set over a bowl. Lightly crush half of them with a fork. Sprinkle with sugar and stir gently to coat. Let sit for one hour.
Set the raspberries aside. Pour the raspberry juices into a 2-cup measuring cup. Add water to measure 1½ cups total.
Pour the raspberry juice mixture into a small saucepan along with the lemon juice. Heat for 3–4 minutes over medium heat until you see bubbles on the edges.
While the raspberry juice is heating, stir the unflavored gelatin with ¼ cup cold water.
Remove the raspberry juice from the heat, and stir in the gelatin mixture. Let cool for 30 minutes.
Place the raspberries in the graham cracker crust. Pour the gelatin/raspberry juice mixture over the top. Transfer to the refrigerator until completely set, about 3–4 hours.
When the gelatin is set, place the heavy cream and powdered sugar in a stand mixer. Beat on medium high until soft peaks form. Spread the whipped cream over the raspberry pie, and refrigerate until ready to serve.
Notes
You can make this pie with black raspberries, too! Make it ahead: you can make the raspberry portion of this pie up to 24 hours in advance. I recommend adding the whipped cream up to 4 hours before you serve it (otherwise it may deflate and get runny). You can also make this with 16 ounces of frozen raspberries. Toss the frozen berries with the sugar, and let them sit for 2 hours at room temperature instead of 1 hour.