Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Thaw the puff pastry according to the package instructions.
Roll out the pastry dough to 1/8-inch thickness and cut into 3-inch squares.
Heat olive oil in a skillet over medium heat.
Add the red onion, garlic, and red bell pepper.
Sauté until softened, approximately 6-8 minutes.
Stir in the tomatoes, peas, flaked tuna, and hard-boiled eggs.
Cook until excess moisture evaporates.
Season with salt and pepper.
Allow the mixture to cool.
Spoon a tablespoon of the cooled filling into the center of each pastry square.
Fold the squares into triangles, pressing the edges with a fork to seal.
Brush the tops with beaten egg for a golden finish.
Place the turnovers on the prepared baking sheet and bake for 20-25 minutes, or until they turn golden brown.