Step 1

Place a rack in center of oven; preheat to 350°. Using a microplane, finely grate zest of 1 lemon into a medium bowl (keep lemon for juicing later).→ Not sure if your oven is the right temperature? You need an oven thermometer.

Step 2

Add 1 cup flour, 1 cup brown sugar, 1½ tsp. cinnamon, and ½ tsp. salt to bowl with zest and toss with your hands to combine and break up any clumps of sugar.

Step 3

Slowly drizzle ½ cup butter into bowl, stirring with a fork, until no dry spots remain and mixture holds together in pebble-size pieces.

Step 4

If you’re using pistachios, chop on cutting board. It’s okay to have some larger and some finer pieces, but try not to leave any nuts whole. Add to bowl with flour mixture and toss to distribute. If you’re using sliced almonds, no need to chop first–just toss them in.

Step 5

Press half of pistachio crumb mixture (about 1½ cups) evenly across bottom of 8x8" pan. Flatten into pan by pressing very firmly with your fingers and palm. This compact layer will bake into a firm, sturdy bottom crust, and it will be much easier to slice into neat bars. Save remaining crumb mixture for later.

Step 6

Bake until nuts are golden brown and crust is visibly darkened in color, 15–20 minutes. Let cool about 10 minutes. This step firms the crust further, and dries it out, so fruit topping won’t make it soggy.

Step 7

Meanwhile, cut zested lemon in half. Working over another medium bowl, insert fork into lemon flesh and twist it to extract juice from one half (fish out and discard any seeds). You should have about 2 Tbsp. juice. Add 12 oz. blackberries, 1 Tbsp. cornstarch, and a pinch of salt; stir with a rubber spatula to coat berries.→ Want to substitute frozen berries for fresh? Read this first.

Step 8

Scrape blackberry mixture over slightly cooled crust and use spatula to distribute fruit into an even layer.

Step 9

Using your hands, sprinkle reserved crumb mixture over berries in an even layer.

Step 10

Bake bars until top of crumble is browned, nuts are toasted, and juices from berries are bubbling just a bit around the edges, 25–30 minutes.

Step 11

Let cool completely before serving. Cut into bars or squares.