In a large pot, dry stir fry onion over medium heat, stirring constantly. Once translucent, add garlic, ginger, and serrano chile if using, and cook until fragrant. Add whole tomatoes plus liquid, water, and rinsed split red lentils. Stir until bubbling and lightly crush the tomatoes with the back of a wooden spoon. Lower heat to a simmer and cover. Stir occasionally until lentils have expanded and the stew has thickened (about 35 minutes) To bloom the spices, in another pot, heat toasted sesame oil and olive oil over medium heat. Add the mustard and stir until fragrant (about 30 seconds). Add the cumin and turmeric and continue to stir until foamy. Pour oil and spice mixture into lentil pot and stir to blend. Add salt and pepper, adjust spices to taste. Serve over brown basmati rice with some fresh chopped cilantro on top!