Preheat the oven to 400 degrees Fahrenheit.
Pat chicken dry with paper towels & heat large cast iron skillet over medium high heat. Salt and pepper both sides of the chicken thighs.
Add olive oil to hot skillet and place chicken thighs in the oil. Flip after 4 minutes or until the chicken releases easily, then cook for 4 more minutes. Remove chicken and set aside.
Add in onions to the skillet and cook (stirring frequently) for 3 minutes, or until translucent. Add in garlic and cook for 30 seconds or until fragrant.
Add in white wine and bouillon, scraping the brown bits off the bottom of the pan. Cook for 3 minutes.
Add in butter, reduce heat to medium and stir until mostly melted. Sprinkle flour over the butter and stir until raw flour bits aren't visible. Add in 2 tsp salt and 1/2 tsp black pepper.
Add frozen vegetables, chicken stock and thyme, and bring to a boil, stirring every once in a while. Cook until thickened, anywhere from 2 to 5 minutes. Remove skillet from heat. Stir in heavy cream and parsley until well combined.
Cube up chicken and mix in well to the contents of the skillet.
Lightly flour a clean countertop. Using a rolling pin, roll out creases in puff pastry on floured surface. Make passes up and down and side to side to make keep a square shape. Carefully place pastry over skillet. Make a "x" in the center of the puff pastry to allow steam to escape.
Mix egg with water in a small prep bowl or even a coffee mug to make an egg wash. Using a pastry brush, brush the egg wash in a thin layer all over the pastry.
Carefully skillet in oven for 40-50 minutes, or until pull pastry is golden brown.