Drain tofu and squeeze between paper towels or clean kitchen towels to remove excess water. Tear into bite-size chunks.
In a bowl, whisk together lemon juice, water, soy sauce, sugar, and 2 tsp cornstarch until sugar completely dissolves. Set aside.
Coat tofu chunks evenly with 1½ tbsp cornstarch, tossing gently to make sure all pieces are covered.
Heat 3 tbsp oil in a large skillet over medium-high heat. Add coated tofu and fry until golden brown, turning to crisp all sides, about 6-8 minutes total.
In a small pan, heat 2 tsp oil over medium heat. Add minced garlic and ginger, cooking for 30 seconds until fragrant.
Add the prepared lemon sauce mixture to the garlic and ginger. Simmer while stirring until sauce thickens, about 2-3 minutes.
Pour the thickened sauce over the crispy tofu and toss gently to coat evenly.
Serve immediately, garnished with chopped green onion and sesame seeds if desired.