Summarize:

Spray a 9-by-9-inch (23-by-23-cm) glass baking dish with nonstick cooking spray. A 9-inch (23-cm) oval casserole dish may be used, instead. If you do not have cooking spray, grease the dish with butter or shortening. Spread the cooking spray, butter, or shortening around the pan using a clean paper towel to coat the dish completely. You should spread out about one-third of your sliced potatoes on the bottom of the dish. The entire bottom should be covered. The potatoes may overlap into more than one layer, but make sure that they are evenly distributed over the bottom of the dish. Evenly spread one-third of your heavy whipping cream over the potatoes in your baking dish. You could use 35% cooking cream if you do not have whipping cream. Slightly shake the dish from side to side after adding the cream to make sure that all the potatoes are coated. Sprinkle 1/4 tsp (1.25 ml) salt, 1/4 tsp (1.25 ml) pepper, 1/2 Tbsp (7.5 ml) parsley, and 1 tsp (5 ml) minced onion over the contents of the baking dish. Distribute these seasonings as evenly as possible. Note that you are essentially using half of your seasonings here. If working with dried parsley instead of fresh, only use 1/2 tsp (2.5 ml). Arrange another layer of sliced potatoes over the seasonings. Make sure that the potatoes are evenly distributed. Use roughly one-third of your potatoes, or half of the potatoes that are left. Pour another 1/2 cup (125 ml) of cream over the potatoes and add the remaining seasonings. Give the dish a slight shake from side to side to evenly distribute the cream. Layer the remaining potatoes on in an even layer before pouring the remaining cream over them. Give the dish one last gentle shake from side to side to settle the ingredients. The potatoes should be tender when pierced with a fork. The top of the potatoes should also be slightly browned. Leave the dish uncovered as it bakes. Remove the finished potatoes from the oven and only let cool for a couple of minutes before serving.
Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Arrange a layer of potatoes in your prepared baking dish. Pour 1/2 cup (125 ml) over the potatoes. Season with salt, pepper, parsley, and minced onion. Create another layer of potatoes. Top with more cream and seasonings. Finish off with the remaining potatoes and cream. Bake for 50 minutes. Serve hot.