Preheat oven to 400°. Line 2 rimmed baking sheets with parchment paper.
In a large skillet, cook beef and onion together over medium-high heat until beef is no longer pink, stirring to break it up, 4 to 6 minutes. Drain excess fat, if desired. Add curry powder and salt, stirring well; cook for 30 seconds more.
In a small bowl, stir together cornstarch and ¾ cup water until smooth. Add cornstarch mixture and raisins to beef mixture, stirring well. Bring to a boil. Cook and stir until thickened, approximately 1 minute. Remove from heat. Let cool slightly.
Place a phyllo dough sheet on a work surface with short end toward you. Lightly brush phyllo with butter. Cover with another phyllo sheet, and brush with butter. (Keep remaining phyllo covered with plastic wrap and a damp towel to prevent drying out.) Using a sharp knife, cut layered phyllo sheets lengthwise into 3 (14×3-inch) strips. Working with 1 layered phyllo strip at a time, place 1 tablespoon beef mixture approximately 1 inch from a corner of strip. Fold corner of strip over filling, forming a triangle. Fold triangle over, forming another triangle and encasing filling. Continue folding strip in this fashion (like a flag). Brush end of strip with butter and press onto triangle to seal. Place triangle seam side down on prepared baking sheets. Brush top with melted butter. Repeat with remaining phyllo sheets and filling.
Bake until edges of beef triangles are golden brown, 10 to 12 minutes. Serve warm or at room temperature with Yogurt-Chutney Dipping Sauce.
Notes
MAKE-AHEAD TIP: Filling can be made a day ahead, placed in an airtight container, and refrigerated. Let come to room temperature before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/curried-beef-triangles/