Heat oil in large, heavy bottomed pot, such as a dutch oven, until smoking.
While oil is heating, pat chuck dry with paper towels. Generously season the beef with salt and pepper.
Sear on all sides. Remove from pot.
Reduce heat to medium, add onions and garlic. Cook until onions are translucent and taking on a light golden color.
Add butter and let melt. Then add flour. Stir and cook until the roux begins to smell nutty. 3-4 minutes.
Add the first cup of beef broth little by little, stirring constantly. Once incorporated, add the remaining broth.
Add chuck and accumulated juices back to pot. Bring to a simmer.
Add dried thyme, bay leaves, and rosemary. Let simmer for at least 3 hours.
One hour before you want to remove the roast, add the carrots, celery, and potatoes. Cook until tender.
Remove the chuck roast and let rest until cool. Shred into small pieces, and add back to vegetables and broth.
Let cool to room temp (~ 1 hour) then chill in fridge at least 2 hours, up to overnight.
Preheat oven to 350 degrees.
Roll out pie crust and place in pie plate.
Add pot roast filling.
Roll out top crust and place on top of pie plate. Crimp, egg wash, and cut vents.
Place pie in freezer for 15-20 minutes.
Place in 350 degeree oven for ~ 40 minutes, until crust is a dark golden color and filling is bubbling.