Rinse the leeks under cold running water. Drain in a large colander and place on a clean work surface. Using a sharp knife, cut into one-inch long pieces. Set aside.
Heat up the oil over medium-high heat. Add mustard seeds and stir-fry for 2-3 minutes.
Now, add leeks and butter. Cook for five minutes, stirring occasionally. Gently crack three eggs and season with dried rosemary, chili flakes, and salt.