Heat oil in Dutch oven over medium heat until shimmering.
Add potatoes and cook, stirring frequently (potatoes may stick to pot), until golden brown in spots, 4 to 5 minutes. Transfer to bowl.
Add cauliflower to now-empty pot and cook, gently stirring occasionally, until golden brown in spots, 4 to 5 minutes. Transfer cauliflower to bowl with potatoes.
Add cumin seeds and asafetida to now-empty pot and cook, stirring constantly, until fragrant and sizzling, 30 to 60 seconds.
Add onion and salt and cook, stirring frequently, until onion is golden brown, 3 to 4 minutes.
Add serrano, garlic, ginger, coriander, amchoor, turmeric, and chile powder and cook, stirring frequently, until fragrant, about 1 minute.
Return potatoes and cauliflower to pot, along with any excess oil from bowl.
Stir in water, scraping up any browned bits. Bring to simmer.
Adjust heat to medium-low, cover, and simmer, gently stirring occasionally, until vegetables have absorbed water, 15 to 18 minutes.
Transfer to shallow bowl, garnish with cilantro, and serve.
