In a small saucepan over medium heat, combine the vegetable stock, soy sauce, and onion powder. Add the Nature's Filets and bring to a simmer. Cover, reduce the heat to medium-low, and gently cook for 20 minutes.
Once tender and fully re-hydrated, drain away the excess liquid and discard or save for another use, such as soup broth or cooking rice. Lightly press the filets to remove any extra moisture, which will make it harder to sear.
When cool enough to handle, toss the filets in flour to evenly coat. Dip in aquafaba, and finally toss with panko, firmly pressing the breadcrumbs in to adhere.
Set a medium skillet over medium-high heat. Add the oil, and when shimmering, add the breaded filets. Cook for 5 - 6 minutes, until golden brown on the bottom, and carefully flip. Cook 5 - 6 minutes longer on the opposite side until perfectly crisp and caramelized all over.
Meanwhile, for the sauce, simply whisk together the ketchup, worcestershire, and soy sauce until smooth.
Serve the hot filets with tonkatsu sauce and a side of finely shredded cabbage, if desired.