Pour lentils into a small bowl. Pick out discard any stems or stones. 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. The lentils and rice don’t need any fussing with, just let them sit.
Bring water or vegetable broth to boil into a large saucepan. Strain lentils into a sieve. Rinse in cold water. Add to broth. Do the same with the brown rice. Toss in 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 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 sea salt and fresh ground pepper.
Stir lentils and rice together gently. Remove bay leaf and season generously with sea salt, fresh ground pepper and optional cumin.
Serve pilaf lavishly topped with onions.