Place the planks in cool water and allow to soak for at least 2 hours or up to overnight.
Heat grill to between 375 and 400ºF.
Brush the trout, inside and out, with olive oil and season with salt and pepper. Place the trout onto the planks, belly down and open. Place the planks over indirect heat and grill until the fish reaches an internal temperature of 120ºF on an instant-read thermometer, 15 to 20 minutes. Remove from the grill and allow to sit for 5 to 10 minutes before serving as is, or deboning and serving.