A staple on the Indian subcontinent, dal spans a whole category of bean dishes. It sometimes has a loose texture, but this version---thick like a stew and bolstered with zucchini---satisfies as a meat-free meal.
In a small bowl, combine mustard seeds, cumin and turmeric. Heat oil in a 4-quart Dutch oven over medium-high. Add combined spices; cook and stir until fragrant and mustard seeds begin to pop, about 1 minute. Add onion. Cook, stirring occasionally, until very tender and beginning to brown, 8 to 10 minutes. Add jalapeño, ginger and garlic; cook and stir until fragrant, about 1 minute more. Transfer half the onion mixture to a small bowl.
To the remaining onion mixture in the pot, add the water, coconut milk, lentils, salt, pepper and coriander. Bring to a boil; reduce heat. Simmer, uncovered, stirring occasionally, until lentils are nearly cooked but still retain some bite, about 15 minutes. Stir in zucchini; continue to simmer until lentils are cooked and zucchini is tender, 5 to 10 minutes more. Squeeze one lemon half over dal and stir in; taste for salt. Cut remaining lemon half into wedges.
Serve dal over rice. Top with reserved onion mixture and desired toppers. Serve with lemon wedges.