Add ground almonds, oats, sugar, and butter to a bowl. Break up the butter into the dry mixture using the tips of your fingers until it resembles coarse breadcrumbs.
Toss in the cinnamon powder, pumpkin seeds if using and salt into the almond mix.
Spread the crumb out evenly on a lined baking tray and bake in a preheated 180 C/360 F oven for 12-15 minutes, or until lightly golden. Remove and set aside.
For the grilled nectarines:
Halve the nectarines and gently remove the stones. Heat a griddle pan or a regular pan and toss in the butter and maple syrup. Place the nectarines cut side down on the pan and turn the heat down. Let them cook for 8-10 minutes or until they soften. If your nectarines were very ripe, keep an eye out so they don't go mushy.
To serve: place a nectarine half face up in a bowl, and top with ice cream and a handful of crumbs.
The crumb can be made in advance and frozen for up to 1 month.