For the filling:
- 2 tablespoons olive oil
- 1 large leek, thoroughly washed and sliced into half-moons
- 2 stalks celery, halved lengthwise and sliced
- 4 ounces baby bella mushrooms, halved and sliced
- 1 pound lean ground beef
- 3 cloves garlic, minced
- 1/2 teaspoon dried thyme
- 1/4 teaspoon dried rosemary
- 3 tablespoons all-purpose flour
- 1 tablespoon tomato paste
- 1 cup Guinness, or another dark beer, or more beef broth
- 1 cup beef broth
- 1 cup frozen peas
- Kosher salt and black pepper
For the white cheddar mashed potatoes:
- 1 1/2 pounds potatoes, peeled and diced
- 1/4 cup milk
- 1/4 cup unsalted butter
- 8 ounces white cheddar cheese, shredded and divided
- 1 egg yolk
- Kosher salt and black pepper