1. Preheat your oven to 375°F (190°C).

2. In a large, oven-safe Dutch oven or heavy-bottomed pot, heat the beef tallow over medium-high heat.

3. Toss the beef cubes with the flour until evenly coated. Brown the beef in batches, about 2-3 minutes per side, until a nice sear forms. Transfer the browned beef to a plate.

4. Reduce the heat to medium and add the onion, carrots, celery, and garlic to the pot. Sauté for 5-7 minutes, or until the vegetables are softened.

5. Pour in the ale and use a wooden spoon to scrape up any browned bits from the bottom of the pot. Bring the mixture to a simmer and let it reduce by half, about 3-4 minutes.

6. Return the browned beef and any accumulated juices to the pot. Add the beef broth, bay leaves, thyme, and red pepper flakes. Season with salt and black pepper to taste.

7. Cover the pot and transfer it to the oven. Bake for 2 hours, or until the beef is very tender.

8. Remove the pot from the oven and discard the bay leaves. Taste and adjust seasoning as needed.

9. Unroll the refrigerated pie crust and place it over the filling, pressing the edges to seal.

10. Bake the pot pie for an additional 30-35 minutes, or until the crust is golden brown and flaky.

11. Let the pot pie cool for 10-15 minutes before slicing and serving.