To make the marinade, whisk together the water, mustard, soy sauce, and spices.
Add tofu cubes to the marinade, coating them completely.
Heat oil in a non-stick frying pan over medium heat. Add the garlic, leeks, carrots, and mushrooms. Mix well, cover pan to retain moisture, and saute until mushrooms are tender and leeks are translucent, about 3 to 5 minutes.
Add marinated tofu and cook until it is firm and starting to turn golden brown, about 5 minutes. (Remember, covering the pan will help to keep the dish moist).
Turn off the heat, add the spinach, and cook until the spinach begins to wilt, about 1 minute. Add a squeeze of fresh lemon juice (~1 teaspoon) for added flavor (optional).