Cut rotlos into medium chunks. Crush garlic cloves, red chili powder, and cumin seeds to a coarse paste in a mortar and pestle.
Heat oil in a kadai. Add mustard seeds; let splutter. Add cumin seeds, sesame seeds, hing, and curry leaves. Stir for 30 seconds.
Add garlic paste, ginger paste, and green chili; cook, stirring, until oil separates, about 2 minutes.
Add spring onion whites; cook until translucent, 2 minutes. Lower heat, add dry spices (red chili, turmeric, coriander, garam masala), stir well.
Add tomatoes and spring onion greens; cook on high heat until tomatoes break down, 3 minutes.
Add sour buttermilk; stir continuously over medium heat until it comes to a gentle simmer.
Add rotlo pieces; stir gently to submerge. Cover and cook on medium-low heat for 3–4 minutes until rotlo softens and absorbs most of the liquid.
Garnish with fresh coriander. Serve immediately with fried green chilies and onion slices.