Blend all mayonnaise ingredients with an immersion blender and add water until it reaches the consistency you want. Set aside.

Preheat oven to 250 degrees Fahrenheit (120 degrees Celsius). In a heavy oven-safe pot or a Dutch oven, heat a tablespoon of olive oil and sear the cheeks or deckle well on both sides. Remove the meat from the pot and set aside.

Place the sliced onions in the same pot. Sauté until translucent, then add beef back in.

Combine the pineapple juice and soy sauce and pour over the meat until almost completely covered. Bring to a simmer, then cover the pot and bake in the oven for two and a half hours or until fork tender. Cool slightly, then shred. Taste and season with salt and pepper as necessary. 

Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius). Remove frozen pizza rounds from the freezer and let them come fully to room temperature.

Combine the tomato paste, ketchup, and brown sugar and brush the pizza rounds with the sauce. Place about a tablespoon of beef in the center. Pinch corners closed to make a hamentaschen shape.

Bake for 20 minutes or until lightly browned. Serve drizzled with onion mayo.