Slice the tenderloin into 6 even sized pieces on a bit of a diagonal to make pouding the pork out easier. Place each piece between a layer of plastic wrap and use a mallet to pound it out to 1/4 inch thickness.
Add the onion powder, garlic powder, salt, and pepper to a small bowl and stir to combine.
Season both sides of the pork.
For the breading:
Add the flour and seasoned salt to a shallow dish and stir to combine.
Whisk the eggs and water together in a second shallow dish.
Add the panko to a third shallow dish.
Dredge each piece of pork in first the flour, then egg, then panko, making sure the pork is fully coated in the breading.
For baking and serving:
Melt the butter and pour onto a large rimmed baking sheet. If your breaded pork will not all fit on one baking sheet, you'll want to divide the butter between two baking sheets.
Place each of the breaded tenderloins in the butter and bake for 8 minutes.
Flip carefully and continue baking for 7 more minutes or until the breading is crisp and the pork has reached 145 degrees internally.
Serve on buns with pickles, onion, and mustard.
Notes
If you end up needing two baking sheets, you may need to add a bit more butter to fully cover the bottom of the baking dish. The butter is what gives these that deep fried texture and flavor, so don't skip it. If you'd prefer to fry these, heat a skillet with a 1/4 inch of oil and fry over medium heat until golden and crispy on each side. Here in the Midwest, restaurants often serve tenderloins that are as large as a dinner plate with a single bun sitting in the center. They're fun to order, but I always trim off the excess to take home and just eat a normal sized sandwich. If you'd like a larger sandwich, just cut the tenderloin into 3 pieces instead of 6 and pound it out into large round shape about 1/4 inch thick.