Place the lentils in a medium saucepan. Cover with 1 3/4 cups water and add 1 tsp of salt. Partially cover and bring to boil over medium high heat. Reduce heat to medium-low and cook until all the water has evaporated and the lentils are softened but not completed cooked through, 20 minutes al dente. Drain cooked lentils and set aside.
Place the rice, stock and 2 tablespoons of butter in a large pot or dutch oven and bring to a boil over high heat. Add the remaining 2 teaspoons of salt and stir well. Cover the pot and reduce heat to low, cooking just until the stock has been absorbed, approximately 10 minutes.
Meanwhile, prepare the dried fruit mixture in a skillet. Melt the remaining 2 tablespoons of butter over medium heat and add the onion. Cook, stirring constantly, until golden brown, about 10 minutes.
Reduce heat to medium-low and add the dates, raisins, cinnamon, saffron water and sprinkle of salt. Stir and cook until the dried fruit has softened, about 3 minutes. Remove from the heat and cover so it doesn't dry out. Reserve 1-2 tablespoons to use as a topping when serving.
Once the rice has absorbed all the water, sprinkle in the cumin and gently mix. Fold in the lentils and dried fruit mixture.
Gently fluff with a fork then scatter some of the reserved dried fruit mixture on top. Serve hot.