Layer chicken thighs, onions, carrots, raisins, and apricots in a slow cooker.
Whisk chicken broth, tomato paste, lemon juice, flour, garlic salt, cumin, ginger, cinnamon, paprika, and pepper together in a bowl. Pour over chicken and vegetables.
Cook on High 4 1/2 hours or on Low for 7 1/2 hours. Shred chicken slightly using 2 forks. Continue cooking 30 minutes.
Bring 3 cups water to a boil in a saucepan; remove from heat and stir in couscous and butter. Cover saucepan and let stand until water is absorbed completely, 5 to 10 minutes. Fluff couscous with a fork. Serve chicken tagine over couscous.