Heat oil in a medium skillet over medium-high heat. Add beef, chopped onion and garlic; cook, stirring often and breaking into small pieces with a wooden spoon until beef is cooked, about 3 minutes. Add tomato paste, cumin, and cayenne. Reduce heat to medium; cook, stirring often, to let flavors meld, about 4 minutes. Add cilantro; season to taste with salt & pepper. Let filling cool to room temperature.
Preheat oven to 375 degrees. Line 2 baking sheets with parchment paper. Cut each pastry sheet into four 4 1/2" squares for a total of 8 squares. Lightly brush edges of squares with glaze. Spoon filling into center of each, dividing equally. Fold edges over, forming triangles, and press to seal. Crimp edges with tines of a fork. Divide triangles between prepared sheets. Brush tops with glaze.
Do Ahead: Can be prepared 6 hours ahead. Cover & chill.
Bake until tops are puffed and golden brown, about 20 minutes.
Meanwhile, puree beans and sour cream in a blender until smooth. Transfer to a medium bowl. Stir in half of tomatoes and half of scallions. Season to taste with salt and pepper. Sprinkle remaining tomatoes and scallions over.