Heat oil in heavy saucepan. Stir in onion, cook over medium heat until translucent. Stir in chicken, 1/2 teaspoon salt and 1/4 teaspoon black pepper; continue to cook, stirring occasionally, until outside is browned. Stir in chicken broth, tomato paste, peanut butter, carrots, bell peppers, curry powder, red pepper flakes, remaining 1 teaspoon salt and 1/4 teaspoon pepper. Bring to a boil, reduce heat; simmer, covered, stirring occasionally, 30 minutes. Stir in sweet potatoes and continue to cook until potatoes are tender and chicken is no longer pink (about 30 minutes). Meanwhile, prepare couscous according to package directions using 1 teaspoon salt. Serve stew over hot couscous.