In a medium sized dutch oven or non-stick pan, combine lentils with 4 cups water, and place over high heat. Bring to a boil, removing any foam that rises to the surface. Reduce heat and simmer until lentils are almost done but still have a bite, about 15 minutes. (The exact time will depend on the variety and age of the lentils, but, generally, cook them for about 5 minutes less than the time stated on the package.)
Add washed and drained rice, 1 teaspoon of the salt and spices to lentils. Bring to a boil, then reduce heat, cover and simmer on lowest setting until rice is fully cooked, about 10 minutes.
While lentils and rice are cooking, heat olive oil in a frying pan over medium-high heat. Add 3 diced onions and remaining 1 teaspoon salt, and sauté, stirring regularly, until a golden-brown color and crisp around the edges, 12-15 minutes. Once rice is done, pour onions and their oil over rice and lentils, give one gentle stir, and place a dish towel or paper towels under the lid and cover. Remove from heat and set aside for at least 10 minutes before serving to let flavors meld.
Meanwhile, prepare the crispy onion topping: Heat vegetable oil in a small, deep skillet or wok until hot. Sprinkle onion half-moon slices with cornstarch and toss to combine. Add seasoned onions to hot oil and fry until crisp and golden-brown, taking care not to burn, about 5 minutes. Remove with a slotted spoon and drain on paper towels until dry and crispy.
To serve, transfer mujadara to a serving platter and top with crispy onions.
* Serve alongside a simple cucumber and tomato salad.