Preheat your oven to 180°C / 350°F.
Heat the olive oil in a large oven-proof skillet over medium heat.
Add the onion into the skillet and cook, stirring frequently, until translucent.
Add the kale and cook, stirring frequently, until wilted.
Season the kale and onions with a bit of salt and pepper.
Scatter roasted pumpkin cubes over the top of the kale and onions, followed by the crumbled goat cheese.
Whisk the eggs in a large bowl together with the milk, if using.
Pour the egg custard over top of the kale and pumpkin, and shake the pan to distribute.
Remove from the stove top and place the frittata into the oven. Bake for 5 - 10 minutes, and then switch to the broil setting.
Broil the frittata for about 5 minutes - but watch it carefully, you'll want the top be puffed and golden, but look away for too long and it will start to burn.
The frittata is finished when it's puffed and golden, and jiggles only a little bit when you shake the pan.
Remove from oven and let cool for a few minutes before slicing.