Heat the oil in a wok or deep skillet over medium-high heat until shimmering. Add cumin seeds and cook until they crackle, about 15 seconds.
Add the sliced onions and sauté until they turn golden and soften, about 5 minutes, filling the air with a sweet aroma.
Stir in the minced garlic and ginger, cooking for about 30 seconds until fragrant and slightly bubbling.
Add chopped tomatoes and cook until they break down, releasing juices and oil begins to separate, about 5-7 minutes.
Toss in the sliced bell peppers and stir well, coating them with the spice mixture. Cook over high heat for 8-10 minutes, stirring often, until they develop slight charring and are tender but still vibrant.
Sprinkle in ground coriander, garam masala, and chili powder, stirring to bloom the spices and release their aroma, cooking for another minute.
Check the consistency; if the mixture is too watery, cook a little longer to thicken and develop a glossy, slightly reduced sauce.
Finish by stirring in chopped fresh coriander and a squeeze of lemon juice if desired, then remove from heat and let rest for 2 minutes.
Serve hot, ideally with rice or flatbread, enjoying the smoky, spicy aroma and vibrant colors.