Nabeyaki Udon Soup with Chicken Spinach and Mushrooms

Name: Nabeyaki Udon Soup with Chicken, Spinach, and Mushrooms
Ingredients: 1/2 cup dried sardines (about 1 ounce), or 1 (5) piece dried kombu; 5 ounces mature spinach (about 3/4 bunch), trimmed; 3/4 teaspoon kosher salt, plus more; 1/2 cup snow peas; 2 boneless, skinless chicken thighs (about 8 ounces), cut into 1 pieces; 7 ounces thinly sliced shiitake, shimeji, or enoki mushrooms, roots trimmed; 2 (1/2-ounce) pieces fried tofu (aburaage), cut into 1/4 slices (optional); 3 tablespoons soy sauce; 1 teaspoon mirin; 8 ounces udon noodles; 1/2 cup sliced scallions; 1 1/2 ounces Japanese fish cake, thinly sliced (optional); 2 large eggs