Preheat the oven to 425°F (220°C). Line two large baking sheets with parchment paper.

To prepare the tofu, whisk together the cornstarch, oil, and soy sauce in a large bowl. Add the tofu and mix gently with a spatula until each piece is coated. Place the tofu onto one baking sheet in an even layer, ensuring that the pieces are not touching.

To prepare the cauliflower, place the florets onto the other baking sheet. Drizzle with oil and sprinkle with salt. Massage the oil in the florets until each piece is coated, then spread the pieces into a single layer, ensuring that the pieces are not touching.

Place the tofu sheet in the oven on the middle rack and the cauliflower sheet on the top rack. Bake for 20 minutes, then flip the tofu and cauliflower pieces to evenly roast both sides. Return to the oven and bake for about 20 minutes, until the cauliflower is tender and charred and the tofu is firm, crisp, and lightly charred. Remove from the oven and keep near the stove.

While the tofu and cauliflower are baking, whisk together the soy sauce, maple syrup, tomato paste, vinegar, sriracha, cornstarch, and pepper in a small bowl with ½ cup (120 ml) water until well incorporated. Leave the bowl near the stove.

Heat a large wok or sauté pan over medium-high heat. Add the oil and heat until it shimmers and easily coats the pan. Add the ginger, garlic, white part of the scallions, and red chilies (if using), and cook until lightly browned, 1 to 2 minutes.

Add the reserved sauce and simmer until bubbly and starting to thicken, about 1 minute.

Add the tofu and cauliflower and stir to coat. Simmer, stirring occasionally, until the sauce thickens, 2 to 3 minutes. Taste and adjust for salt.

Garnish with the green part of the scallions and serve hot with rice, if desired.