After soaking lentils and rice (see notes), strain them through a sieve. Rinse in cold water.
Bring vegetable broth to a boil in a large saucepan. Add the lentils and rice to the broth. Toss in the bay leaf. Reduce heat to low and cook, covered, for 30 to 40 minutes, until lentils and rice are soft and fluffy and have soaked up all the liquid. Remove cover, remove from heat and set aside.
Just before serving, heat the oil in a large skillet over medium-high heat. Add sliced onion and cook, stirring, for 3 to 5 minutes, until onions start to soften and turn golden and fragrant. Reduce heat to medium, and cook, stirring, another 10 minutes or so, until onions are brown and tender. Season with salt and pepper.
Stir lentils and rice together gently. Remove bay leaf and season the rice and lentil mixture generously with salt, pepper and cumin to taste. Serve the pilaf topped with the cooked onions.
Notes
Pour lentils into a small bowl. Cover with cold water. Do the same with the rice, in a separate bowl. Leave 'em to soak for 30 minutes at a minimum, for 2 hours, if you've got the time.