Heat 2 tablespoons peanut oil in a frypan; fry the cumin seeds, mustard seeds, and red chillies in the hot oil until the seeds begin to splutter, 2 to 3 minutes. Add the curry leaves, turmeric and asafoetida powder; fry another 30 seconds. Stir the potatoes into the mixture to coat; cook until they begin to brown, about 5 minutes. Season with salt. Sprinkle the water over the potatoes; cover the frypan with a lid and cook until the potatoes are cooked, but still firm, about 10 minutes.
Meanwhile, heat 1 tablespoon peanut oil in a separate frypan; cook the spinach in the hot oil until wilted. Sprinkle with salt; continue cooking until all the excess moisture evaporates, 2 to 3 minutes. Stir into the potato mixture. Season with the chilli and cumin; stir. Cook until the spices integrate, 5 to 10 minutes. Serve hot.
Asafoetida powder is a spice made of mature fennel and can be found in shops specialising in Indian cooking.