In a small bowl whisk together ⅓ cup water, red pepper flakes, soy sauce, rice vinegar, cornstarch and oyster sauce, if using. Make sure the cornstarch dissolves.
Line a large plate with a folded dish towel or two layers of paper towels. Place the tofu in a single layer on the towels to drain.
Heat 2 tablespoons of the oil in a 14-inch flat-bottomed wok or large nonstick frying pan over medium-high heat until shimmering. Add the tofu, spread in a single layer, and season generously with salt. Cook, flipping rarely, until golden-brown on at least 2 sides, about 10 minutes total. Be careful. It might splatter. Remove to a clean plate.
Place the remaining 2 tablespoons of oil and the eggplant in the pan. Season with salt and stir-fry until charred in spots, about 2 minutes. Add the garlic, bok choy and carrots and cook until fragrant, about 30 seconds. Add the reserved tofu and stir to combine.
Whisk the reserved sauce to recombine and dissolve the cornstarch; then add to the pan. Reduce the heat to medium and stir-fry until the sauce has thickened and coats the tofu and eggplant, about 1 minute more. Remove from the heat, add the cilantro and scallions and toss to combine. Serve immediately on its own or over quinoa.