Place tomatoes, onion, garlic, raisins (if using), brown sugar and lemon juice in a slow cooker and stir to combine. Sprinkle brisket with 1 1/2 tsp. salt and season with pepper. Place brisket on tomato mixture and then spoon half of tomato mixture on meat. Cover and cook on high until meat is fork tender, 5 to 6 hours.
Transfer brisket to a cutting board, tent with foil to keep warm, and let stand for 10 minutes. Skim fat from sauce and discard; season sauce with salt and pepper. Slice brisket across grain, transfer to a serving platter and spoon some sauce over meat.