Heat oil in a non‑stick pan. Add mustard seeds to sizzle.
Toss in curry leaves, ginger & garlic, onions, and tomatoes. Sauté for 5-7 minutes until fragrant.
When the onions become softer, combine the green leeks, green chili, salt & pepper, turmeric powder, chili powder, and chili flakes. Mix everything together and continue cooking for 5 minutes.
Add the tuna fish. Mix gently if you prefer some fish pieces intact. I like it mixed well even if the fish becomes flaky.
Continue cooking for 8-10 minutes. Remove from stove and pair with your meals.