Heat oven to 300°F. Season meat with salt and pepper all over.
Combine flour, espresso powder, cloves and allspice in a small bowl. Coat the meat with the rub and shake off any excess. Reserve 3 tablespoons of the spiced flour for your gravy.
Heat a large Dutch oven over medium-high, add beef and brown evenly all over, about 15-20 minutes. Remove to a plate and add carrots, celery, onion, herb bundle, salt and pepper to pan and cover. Sweat veggies 8-10 minutes, stirring occasionally. Add cider and bring to bubble; add stock and beef, and cover. Transfer to oven and roast 4 hours. Remove meat from pan and cover with foil to rest.
Rinse and dry the potatoes. Heat oil in large, cast-iron or deep skillet over medium-high to high heat. When oil is hot, add potatoes and season aggressively. Cook, tossing often with tongs, until crispy and golden brown all over. Add apples, leeks and thyme, and toss with tongs, Lower heat a bit, cover and cook 5-8 minutes to tender-crisp.
For the gravy, strain liquid from the Dutch oven and push solids through the strainer with a wooden spoon to enrich the sauce. Melt butter in the pot or add oil and whisk in reserved, seasoned flour. When roux bubbles, whisk in strained liquid and thicken into gravy. Add Worcestershire sauce to taste.
Slice brisket and serve with gravy and potatoes with apples and leeks alongside.