Soba Noodles with Spinach and Tofu

Ingredients

• 1 pkg (250 g) Japanese buckwheat (soba) noodles
• 1 lb fresh spinach trimmed
• 1 pkg (10 oz/300 g) soft tofu cubed
• 2 green onions thinly sliced on bias
• 2 tablespoons thin strips nori (optional)
• 1 tablespoon toasted white sesame seeds
• 1 piece (about 4 inches/10 cm long) dried kelp (konbu)
• 2 cups water
• 1 cup bonito flakes
• 1/2 cup mirin
• 1/3 cup sodium-reduced soy sauce
• 2 tablespoons granulated sugar
• 1 tablespoon sake