Heat 2 tablespoons oil in a large pot over medium-high heat. Add cumin seeds; cook until turn golden brown and begin to pop, a few seconds; reduce heat to medium. Add onion; cook until lightly browned. Stir in serrano chiles and ginger paste; cook 1 minute. Season with coriander, turmeric, cayenne pepper, garam masala, and paprika; cook until fragrant, 30 seconds.

Add potatoes and salt to the pot; cover and cook for 5 to 7 minutes. Add cauliflower; cover and steam in its own juices until tender, about 20 minutes. Stir in lemon juice. Add remaining 1 tablespoon oil around edges of the pot. Increase heat to medium-high; cook until browned, 3 to 5 minutes, stirring gently to avoid mashing cauliflower.