Heat oven to 400 degrees.

Melt butter in a large saute pan over medium heat. Add onion and salt and cook, stirring occasionally, for about 30 minutes.

Add garlic, thyme, and sage along with a couple pinches of salt and pepper. Cook, stirring occasionally for 2 minutes. Sprinkle in flour and cook, stirring frequently, for 1 minute.

Deglaze with a few splashes of white wine, then slowly pour in the beef broth while scraping all the yummy bits off the bottom of the pan.

Stir in onion soup mix, heavy cream, frozen veggies, shredded chicken, and a few large pinches of salt and pepper.

Turn heat to medium-high and bring to a simmer. Reduce heat and simmer gently for 3-5 minutes or until it thickens. Stir occasionally. Season to taste with salt and pepper.

Transfer filling to an 11.5 inch deep dish pie pan, 9×13 white baking dish, or keep it in the large saute pan.

Unroll puff pastry on a lightly floured surface. Use a rolling pin to roll it out slightly into a square and flatten the seams. Use a pizza cutter to slice puff pastry into 2" squares.

Layer the puff pastry squares over the pot pie, starting around the edge and working towards the center, overlapping the pieces slightly until you’ve used all of them.

Beat egg with a splash of water. Brush the puff pastry with egg wash.

Bake for 25-30 minutes, or until the puff pastry is golden brown and flaky.

Let the pot pie cool for 5–10 minutes before serving.