Heat 2 tablespoons peanut oil in a skillet; fry the cumin seeds, mustard seeds, and red chile peppers 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 skillet with a lid and cook until the potatoes are cooked, but still firm, about 10 minutes.
Heat 1 tablespoon peanut oil in a separate skillet; 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 ground red pepper and cumin; stir. Cook until the spices integrate, 5 to 10 minutes.