Combine the lentils, tomato, 2 cloves of garlic, ginger, serrano chile, turmeric and 6 cups water in a large saucepan. Bring to a high simmer and cook, occasionally whisking vigorously, until the lentils are tender and begin to fall apart and the mixture is like a thin porridge, 15 to 18 minutes. Add the spinach to the pan in batches and stir until wilted. Add the lemon juice and 1 1/2 teaspoons of salt. Remove from the heat.
Melt the butter in a small skillet over medium heat. Add the remaining clove of garlic and the cumin seeds and mustard seeds. Swirl until the garlic and is golden and fragrant, about 3 minutes.
Pour the lentils into a large serving bowl and drizzle with the garlic butter. Serve hot.
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