Mangkuk Nasi Tahu Teriyaki

Bahan-bahan (4 porsi)

• 14 oz . firm tofu ((1 package, 400g))
• 1/4 cup cornstarch ((plus 2 teaspoons))
• ⅓ cup minyak sayur
• 1 teaspoon ginger ((freshly grated))
• 1 medium onion ((thinly sliced))
• 2 tablespoons mirin
• 2 tablespoons kecap
• 2 teaspoons kecap hitam
• 1 tablespoon gula merah
• 1 teaspoon sesame oil
• 1/2 cup air
• 1 scallion ((chopped, optional))
• Steamed rice ((to serve))
• Toasted sesame seeds ((optional))