To make the ice cream: In a food processor fitted with the steel blade, combine the heavy cream, cream cheese, sour cream, granulated sugar, lemon zest and juice, and a pinch of salt. Blend until smooth and creamy, and the sugar is completed dissolved, 2 to 3 minutes. Transfer to a plastic container or loaf pan; cover and freeze until solid, about 8 hours or overnight.
To make the crumble: Preheat oven to 325 degrees and set an oven rack in the middle position. Line a rimmed 13- by 18-inch baking sheet with parchment paper.
In a food processor fitted with the steel blade, combine graham crackers, pecans, brown sugar, butter and pinch of salt; pulse until finely chopped. It's fine to leave some small chunks; just be sure the brown sugar is incorporated.
Transfer mixture to the prepared baking sheet and press into a compact layer about 1/8 inch thick, like a crust (don't worry if the edges are uneven). Bake until golden around the edges, 12 to 15 minutes; let cool to room temperature, then break into chunks. (Or, instead of making a crumble, you can break the crust into larger shards and serve the pieces as a "cookie" garnish.)
Scoop ice cream into bowls and top with crumble. The ice cream will keep for a few weeks in the freezer, and the crumble will keep in an airtight container at room temperature for up to a week.
Nutrition information per ½ cup serving: