Measure dashi powder into a large mixing bowl or measuring container, and add 4 cups (1 L) of boiling water. Stir to dissolve, then pour into a saucepan.
Add rice wine, miso paste and ginger. Bring to a boil, then reduce heat to a gentle simmer. Add diced tomatoes and simmer for a further 10 minutes.
Add spinach leaves and soy sauce, and cook for a minute until leaves start to wilt. Divide cubed tofu equally among four soup bowls and ladle broth overtop.
Recipe Notes
Per serving: 116 calories, 10 g protein, 4 g fat (1 g saturated fat), 11 g carbohydrates, 2 g fibre, 0 mg cholesterol, 820 mg sodium