Sprinkle the brisket with 1 teaspoon salt and 1/2 teaspoon pepper and cut crosswise into 3 pieces. Heat the olive oil in a large skillet over medium-high heat, add the brisket in batches and brown, about 3 minutes per side. Transfer to a 5-to-6-quart slow cooker.
Add the tomato paste to the skillet and cook, stirring, 1 minute. Whisk in the wine, vinegar and pickling spice sachet and simmer 4 minutes. Add to the slow cooker along with the beef broth, carrots, celery, leeks and half of the gingersnaps. Cover and cook on low, 8 hours.
Remove the meat, vegetables and spice sachet. Whisk the remaining gingersnaps into the cooking liquid and add salt, pepper and parsley to taste. Set aside 1 piece of brisket, 1 cup vegetables and 2 cups gravy to make Corn, Squash and Beef Casserole. Slice the remaining meat and serve with the vegetables and gravy.