Step 1

Line two rimmed baking sheets with parchment paper. Pulse 2 1/2 cups flour and 1 1/2 teaspoons salt in a food processor until combined, 1 or 2 times. Add butter and pulse until butter is the size of peas, 8 to 10 times. Transfer mixture to a bowl. Add 6 tablespoons ice water and stir with your fingertips until no dry patches of flour remain and dough holds together when squeezed (add additional water, 1 tablespoon at a time, if needed). Divide dough in half; flatten halves into disks and wrap in plastic. Chill at least 2 hours or up to overnight.

Step 2

Working with one disk at a time, roll dough to 1/8-inch thickness on a floured surface. Cut out 5 rounds, using a 5-inch round cookie cutter, rerolling scraps once, if necessary. Repeat with second disk. Transfer rounds to prepared baking sheets; refrigerate 30 minutes.

Step 3

Whisk together granulated sugar, lemon zest, remaining 3 tablespoons flour, and remaining pinch salt in a bowl. Add strawberries and rhubarb; toss to coat. Spoon filling into center of dough rounds (about 1/4 cup each), leaving a 3/4-inch border. Brush edges of dough with egg. Fold dough onto filling, overlapping it as you go. Lightly brush dough with egg; sprinkle with demerara sugar, dividing evenly. Freeze 30 minutes.

Step 4

Preheat oven to 425°F with oven rack in lowest position. Bake until golden brown and filling is bubbly, 25 to 30 minutes. Brush fruit with jelly. Serve warm or at room temperature.