In a large pot, heat sesame oil over medium heat. Add minced garlic and grated ginger, sauté for 1 minute until fragrant.
Pour in the broth and dashi stock (if using). Stir in soy sauce and mirin. Simmer for 10 minutes.
In a separate pot, boil water and cook udon noodles according to package instructions (usually 8-10 mins for dried, 2-3 mins for fresh). Drain and rinse under cold water.
Add shiitake mushrooms to the broth and cook for 3-4 minutes.
Stir in baby spinach and tofu (if using) until wilted (about 1-2 minutes).
Divide cooked udon noodles into bowls. Ladle the hot broth and vegetables over the noodles.
Top with green onions, soft-boiled eggs, and a drizzle of chili oil (optional).
Enjoy hot with chopsticks and a spoon!
