I was first introduced to Muhajadarrah over ten years ago with this recipe, from Deborah Madison's Vegetarian Cooking for Everyone, and I haven't strayed from it since. No need to, really. But a quick google search reveals that, like most classic dishes, there are dozens of different recipes and variations. You can certainly introduce middle eastern spices such as cumin or cinnamon, but I recommend trying this first just as it is. It's revolutionary to taste something so delicious and satisfying that comes from using the simplest and most humble ingredients imaginable.
A few bits of advise: be sure to caramelize the onion very deeply and to use plenty of salt and pepper. These are your only seasonings, so they're very important. Also, be sure to use brown or green lentils, not the orange ones which will just dissolve to a mush.
6 T olive oil
1 very large onion (or two medium) sliced into rounds 1/4 inch thick
1 1/4 cups green or brown lentils, sorted and rinsed
3/4 cup white or brown long-grain rice
salt and freshly ground pepper
Heat the oil in a large skillet over medium heat. Add the onion and cook, stirring frequently, until it becomes a rich, dark brown color, about 12-15 minutes. Meanwhile, put the lentils into a saucepan with one quart of water. Bring to a boil and simmer 15 minutes. Add rice, plenty of pepper and more water, if needed, to cover. Cover and cook over low heat until rice is done, about 15 minutes. Stir in half the onions and about 1 teaspoon of salt, then cover and let stand off the heat for 5 minutes. Spoon the lentil-rice mixture on to a platter and cover with the remaining onions.
I serve this with a bowl of cubed feta cheese or good, thick yogurt; some jarred roasted red peppers tossed with sherry vinegar, olive oil and a little garlic; and Trader Joe's middle eastern flat bread, heated in the toaster oven until it puffs. Occasionally I go all out and make a tangy little salad of parsley, capers, shallots, lemon, s & p.