Prepare and preheat your grill by building a fire on 1 side of the grill.
In a small bowl, mix together the salt, pepper, red chili powder, and herbes de Provence. Open each trout's belly and lightly rub on the chili spice mixture. Next, place 2 lemon slices into each trout's belly and then carefully place each trout onto a cedar plank.
Cook the trout planks in a covered barbecue away from direct heat for 15 minutes. Then, move the planks over the direct heat and cook for an additional 5 to 7 minutes, covered. The cedar planks will begin to smoke, and the trout will begin to turn golden brown. Once the trout is finished cooking, remove the planks from the grill and set aside. Carefully, lift the fish off the plank and serve.