Heat oil in a sauté pan over medium heat. Add onion and sauté until extra soft and golden, about 10 minutes.
Add pastrami and sauté for 1 to 2 additional minutes. Stir in mayonnaise, ketchup, honey, hot sauce, and soy sauce.
Place 1 tsp filling in the center of each wrapper. Brush cornstarch mixture around the edges (this will help seal the hamantaschen). Fold into hamantaschen shape.
To bake the hamantaschen, preheat oven to 400°F. Place hamantaschen on a lined baking sheet and spray the tops with nonstick cooking spray. Bake for 8 to 10 minutes.
Alternatively, you can fry the hamantaschen (of course, it's much tastier this way and that's the version I prefer). Heat 1 inch oil in a sauté pan over medium-high heat. When hot, add hamantaschen and fry until golden and crispy, about 3 to 4 minutes per side.