Cook soba according to package directions; drain. Return to hot pan; cover and keep warm.
Meanwhile, in a large skillet heat sesame oil over medium-high heat. Add garlic and ginger; cook and stir for 30 seconds. Add sweet pepper; cook and stir for 2 minutes. Add tofu; cook for 1 minute more, stirring gently.
Stir in cilantro (if desired), teriyaki sauce, and the water. Gradually add spinach, stirring just until spinach is wilted. Serve with cooked soba.