Preheat the oven to 350°F. Place the tofu on a large rimmed baking sheet with 1 tablespoon of the sesame oil and the tamari, flipping the planks to make sure they're well coated. Sprinkle with black pepper. Arrange the tofu slices in a single layer. Cover tightly with aluminum foil and bake for 20 minutes.
Remove the foil and flip the tofu slices, pushing them to one side. Add the bok choy and drizzle with the remaining 1 tablespoon sesame oil. Sprinkle with salt. Bake for 15 minutes, uncovered, until the tofu is browned and the bok choy is wilted and crisped at the edges.
While the tofu is baking, combine the peanut butter, ½ cup water, salt, lime juice, ginger, hoisin sauce, and red pepper flakes in a blender. Blend until smooth. Use additional water to thin the sauce if necessary.
Divide the tofu and bok choy among plates. Drizzle with peanut sauce, garnish with scallions and red pepper flakes if desired.
