In a large skillet over medium-high heat, brown brisket on both sides in 2 tablespoons olive oil. Transfer meat and drippings to a 5-qt. slow cooker. Combine the peanut butter, soy sauce, sesame oil, cilantro, lemon juice, garlic, pepper flakes and pepper; pour over brisket. Cover and cook on low for 8-9 hours or until meat is tender.
Remove brisket and keep warm. Combine cornstarch and water until smooth; stir into cooking juices. Cover and cook on high for 30 minutes or until thickened. Meanwhile, in a large skillet or wok, stir-fry the carrots, peppers and onions in remaining olive oil until crisp-tender. Add peanuts if desired. Stir cooking juices and stir into vegetable mixture.
Thinly slice meat across the grain. Place rice on a large serving platter; top with meat and vegetable mixture.