Place ricotta into a fine mesh strainer set over a bowl. Allow to drain for at least 1 hour before using.

Line an 8x8-inch square pan with enough parchment paper to have overhang on all sides.

Combine butter, brown sugar, salt, and nutmeg in a microwave-safe bowl. Microwave, stopping to stir every 30 seconds, until mixture begins to foam and sugar is mostly dissolved (about 2 minutes total). Stir in graham cracker crumbs, pistachios, and almond extract until evenly moistened.

Press crust mixture firmly and evenly into the bottom of the prepared pan. Freeze crust until chilled and set, about 20 minutes.

Combine drained ricotta and cream cheese in a large bowl. Beat with an electric mixer on medium speed until smooth. Add in confectioners sugar, lemon zest, lemon juice, vanilla, and salt. Beat on medium speed until smooth and combined.

Remove crust from freezer. Pour filling over crust and smooth into an even layer. Return pan to the freezer and chill until filling is set, at least 3 hours.

To cut bars, remove from the pan using the parchment overhang and place onto a cutting board. Run a sharp knife under hot water, dry it, and cut bars. Wipe knife between each cut, reheating and drying as needed.

Sprinkle bars with chopped pistachios before serving.