How to Make It
In a large bowl, toss the mustard greens with 1 teaspoon of salt. Gently massage the greens until just starting to wilt. Let stand at room temperature for 20 minutes, tossing occasionally. Gently squeeze the excess water from the mustard greens and drain well. Transfer the greens to a 1-pint glass jar, cover and let stand at room temperature for at least 4 hours or overnight.
Meanwhile, in a large saucepan, cover the chicken and kelp with 12 cups of water. Add a generous pinch of salt and bring to a boil. Simmer over low heat until the broth is richly flavored, about 2 1/2 hours, skimming off the foam as it rises to the surface. Strain the broth through a fine sieve into a heatproof bowl; you should have about 4 cups. Reserve the chicken for another use. Return the broth to the saucepan, cover and keep hot.
In a small bowl, whisk the Fresno chiles with the fish sauce, lime juice and mala sauce. Season lightly with salt.
In a large saucepan of boiling water, cook the udon until tender, about 2 minutes. Drain well and transfer to 4 warm bowls. Ladle the hot broth over the udon and top with some of the preserved mustard greens. Serve hot, passing the chile sauce at the table.
Mala sauce is a Chinese chile sauce with Sichuan peppercorns and spices. It is available at Asian markets or online.