Bake the puff pastry according to the package directions. Once baked, cut it into 4 equal portions.
In a large skillet over medium heat, melt the butter. Add the diced onion and cook until softened and lightly browned.
Whisk the cornstarch with 60 ml of the cold chicken stock until smooth.
Add the remaining 240 ml chicken stock, fully cooked shredded chicken, milk, frozen vegetables, rosemary, salt, half & half, garlic, black pepper, and paprika. Bring to a gentle simmer over medium-low heat and cook until the vegetables are heated through.
Stir the slurry again, then pour it in gradually while stirring. Simmer until the filling thickens, then remove and discard the rosemary sprigs.
Divide the filling among 4 bowls or ramekins and top each with one pastry portion.
Enjoy!