1. Sprinkle brisket evenly with 1 teaspoon salt and pepper. Heat a large skillet over medium-high heat. Add oil; swirl to coat. Add brisket; cook 5 minutes, turning to brown on all sides. Rub brisket with garlic powder and paprika. Arrange carrots and celery in a 6-quart electric slow cooker coated with cooking spray; top with brisket, fat cap side up.
2. Return skillet to medium heat. Add onions to pan; cover and cook 10 minutes, stirring occasionally. Uncover. Stir in garlic; cook 5 minutes or until onions are tender and golden. Arrange onion mixture over brisket.
3. Combine remaining 1/4 teaspoon salt, stock, tomatoes, brown sugar, and vinegar in hot skillet, stirring with a whisk to loosen browned bits. Pour tomato mixture around brisket. Place thyme and bay leaves in slow cooker, pressing into tomato mixture. Cover and cook on LOW for 7 hours or until brisket is tender when sliced. Cool slightly in cooker, about 1 hour.
4. Place brisket on a cutting board. Trim fat cap; discard fat. Cut brisket across the grain into thin slices. Pour sauce through a sieve over a bowl; discard carrots, celery, thyme, and bay leaves. Return onions to sauce. Serve brisket with sauce. Garnish with fresh parsley, if desired.