Place flour, sugar, and salt in a large bowl and use your fingers or a pastry cutter to work butter into flour until butter is in pea-sized clumps. Drizzle 4 tablespoons ice water over flour and gently toss until dough comes together into a shaggy mass (if dough seems too dry, drizzle in an extra tablespoon or two).
Turn dough onto a surface and very gently knead several times until dough comes together. Lightly flour surface and roll dough out into a 4x6 rectangle. Fold dough into thirds and wrap in plastic wrap. Chill dough in refrigerator 2 hours or up to 6 days.
Blueberry Filling
Combine blueberries, honey, lemon juice, and cornstarch mixture in a small saucepan. Bring to a simmer over medium heat and cook until blueberries are broken down and thickened, stirring occasionally. Allow filling to cool.
Preheat oven to 400F and line a large sheet pan with parchment paper. Set aside.
Remove pastry dough from refrigerator and turn onto a floured surface. Keep dough folded into thirds and roll into a 10-inch square. Use a knife or pizza cutter to cut into 9 even squares.
Place about 1/2-1 tablespoon blueberry filling in each square and fold over into a triangle shape. Use a fork to seal and crimp edges. Place each turnover on prepared sheet pan. Chill turnovers in freezer 15 minutes.
Remove turnovers from freezer, brush with egg white mixture, and sprinkle with coarse sugar. Bake turnovers at 400F 15-20 minutes until golden brown.
Allow turnovers to cool on sheet pan 10 minutes, then transfer to a wire cooling rack. Enjoy!
Notes
Storing InstructionsStore turnovers in an airtight container at room temperature up to 4 days. Turnovers may also be frozen up to 2 months.