Heat the 2 ½ TBSP oil in a 12-inch deep sauté pan or Dutch oven over medium-high heat. Once it shimmers, add cinnamon stick and cumin seeds and toast for 1 to 1 ½ minutes, stirring or shaking the pan frequently, until darker in color and aromatic. Add the onions, season with a pinch of salt, and cook until nicely browned, 7 to 8 minutes.
Add in the garlic, ginger, turmeric, and asafoetida, if using. Cook for 1 to 2 minutes, stirring very frequently.
Add the amchur if using, coriander, chile powder, serrano pepper, tomatoes, salt, and crack in some pepper. Lower the heat as needed if the spices start to stick. Stir well, using the tomato juice to scrape up the browned bits. Simmer for 4 to 5 minutes, or until the tomatoes have softened and almost melted, and the oil starts to separate from the tomatoes.
Add roasted cauliflower and potatoes plus any browned bits on the pans. Gently mix to coat in the masala and cook, uncovered, for about 5 minutes. Toss occasionally, until potatoes and cauliflower are well-coated and tender but not soggy, lowering the heat if it starts to stick.
Crush the kasuri methi with your hands. Add them into the pan, along with the garam masala and butter, if using. Toss to coat briefly and to melt the butter, if using. Turn off the heat and rest for 5 minutes.
Meanwhile, chop the cilantro. Add the cilantro and a couple squeezes of lime or lemon juice to the aloo gobi (more if you didn’t use amchur). Season to taste with salt and more garam masala if desired. Remove the cinnamon stick pieces.