Finely chop the onions and garlic, and sauté them on medium heat in a dash of oil. Stir occasionally. After frying, spread them over the bottom of the baking dish.
Make sure you have potatoes of about equal size for the best effect. Peel and cut them into equal thin slices. Separate them into even piles. I used purple and orange sweet potatoes as well as regular waxy potatoes for this recipe.
Now this is a fun one. Arrange the potato slices in a rainbow pattern in the baking dish, alternating between colours. Some potato slices will be left over; chop them finely and spread them on the bottom of the baking dish.
Neatly arrange the potato slices in the dish, and brush them with melted butter. Cover with aluminium foil and bake in a convection oven at 200 °C for 45 minutes, then remove the foil and bake for an additional 15 to 20 minutes.
Garnish with a beautiful rose of avocado slices. Avocado provides healthy fats and its creamy flavour complements the sweet potatoes perfectly. Serve with some creme fraiche with salt and pepper. Finally, sprinkle fresh thyme leaves over the colourful sweet potato casserole.