Transfer the cubed tofu to a large bowl with 2 tablespoons cornstarch and 1 teaspoon kosher salt. Toss gently to coat, being careful not to break the tofu too much. Set aside.
Make the sauce:
Whisk together all the ingredients for the sauce in a small bowl until smooth. Set aside.
Fry the tofu:
Heat 2 tablespoons neutral oil in a wide pot over medium heat. Once hot, add the tofu in an even layer. Fry in batches for 3 to 5 minutes per side, only flipping the tofu when it’s golden brown and releases easily. Transfer the cooked tofu to a plate as each piece finishes frying. Adjust the heat as needed to prevent the tofu from burning.
Sauté the mushrooms:
Return the heat to medium. Add the mushrooms and cook, stirring often, for 5 minutes until golden brown. Season lightly with salt.
Add the diced shallot, minced garlic, and minced ginger to the mushrooms and sauté for 1 minute until fragrant.
Cook the yu choy:
Add the yu choy to the mushrooms and sauté until wilted. Season lightly with salt. Add the minced white parts of the scallions and cook for 1 minute.
Simmer the sauce:
Add the sauce to the pot and bring to a boil. Reduce the heat to low and simmer for 2 to 3 minutes until thickened. Taste and adjust the seasonings to your preference.
Finish the tofu:
Add the tofu to the pot and toss quickly to coat. Remove from the heat.
To serve:
Spoon the tofu and yu choy over cooked white rice. Serve with the reserved minced scallion greens and a sprinkle of sesame seeds. Enjoy!