Mix in the eggplant, tomatoes, tomato sauce and stock cube and gently simmer covered on low - medium heat for about 20 minutes. Check occasionally that the mixture does not become too dry. If it reduces too quickly and becomes dry, add a little water to keep it moist. Shakshuka should be a "dry-ish" sauce so resist the temptation to add too much water. The liquid from the tomatoes and the tomato sauce should provide enough moisture.