Thaw the puff pastry sheet in the fridge overnight, or at room temperature for 30–40 minutes. Then cut into 4 squares.
Cut the bell pepper, zucchini, and red onion into thin strips. Press lightly with paper towels to remove excess moisture, then toss with garlic powder, salt, pepper, and a drizzle of olive oil.
Combine cream cheese, egg yolk, and garlic powder in a small bowl and mix until smooth and creamy.
Place the pastry squares on a lined baking sheet and prick the centers all over with a fork. Spread the cream cheese filling generously over the pricked area, then arrange the vegetables and ham on top.
Beat the reserved egg white until smooth and brush over the edges of each tart.
Bake at 400°F (205°C) for 16–18 minutes until puffed and golden brown. Let cool for a few minutes before serving.