Heat the vegetable oil in a large skillet over medium heat. Add the cumin seeds and let them sizzle for about 30 seconds until fragrant.
Add the chopped onion and sauté until it turns golden brown, about 5-7 minutes.
Stir in the minced garlic, grated ginger, and slit green chilies. Cook for another 2 minutes until aromatic.
Add the turmeric, coriander, and cumin powders. Mix well and cook for 1-2 minutes to toast the spices.
Pour in the drained diced tomatoes and cook for about 5 minutes, stirring occasionally, until the mixture thickens and the oil starts to separate.
Add the cauliflower florets and salt. Mix to coat the cauliflower with the spice mixture. Cover and cook for 10-12 minutes, stirring occasionally, until the cauliflower is tender but not mushy.
Stir in the garam masala and cook for an additional 2 minutes.
Remove from heat, drizzle with lemon juice, and garnish with chopped cilantro before serving.