The trick to crispy oven-baked potatoes is to soak them in water first. Place the sweet potato rounds in a large mixing bowl, then fill with water until submerged. Let them soak for at least 30 minutes in the refrigerator.  Once done, drain the water and dry the potatoes completely with a paper towel. Then coat with oil and seasoning mixture as usual. Once they're fork-tender, turn the broiler on for 2-3 minutes, watching closely so they don't burn.  It also helps to slice the potatoes on the thinner side, because the middle will usually still be soft.