Heat a saucepan over medium heat. Toast cumin seeds until fragrant, about 1 minute, swirling pan. Set aside. Heat oil and add onion, garlic, and ginger. Cook, stirring, until onions are tender, about 6 minutes. Add turmeric and toasted cumin seeds. Cook 1 minute. Add water and lentils. Bring to a boil; reduce to a simmer and partially cover. Cook until lentils are tender and mixture thickens, about 15 minutes. Season with salt.
Meanwhile, prepare garnish: Combine onion, cucumber, chile, cilantro, and vinegar. Season with salt.
To serve, ladle lentils over rice; top with yogurt and cucumber mixture. Sprinkle with cilantro leaves.