Place sliced potatoes, onion, cubed ham, salt and vegetable broth into the pressure cooker pot. Set cooker to steam, low pressure for 2 minutes.
Do a quick release of pressure after 2 minutes. Open the pressure cooker and transfer the potatoes, onion, and ham to an oven safe, 3 quart casserole which has been sprayed generously with cooking spray., keeping as much liquid in the pot as possible.
Turn the electric pressure cooker to saute, add the half and half, thyme, onion powder, and most of the grated cheese, reserving some to sprinkle on top of the casserole. Stir to melt the cheese.
Once the cheese is melted, pour the cheese sauce over the potatoes, add black pepper if desired, then sprinkle the remaining cheese over the top.
Place in the oven on broil with the top of the carrerole 4 or 5 inches under the element. Broil 12 to 15 minutes until nicely browned.
Remove from the oven and let rest 5 to 10 minutes before serving.