Preheat the oven to 250'C, if it goes up that high, or at least 220'C. Put a baking sheet in to heat up.
Meanwhile heat the oil in a frying pan over a medium heat and add the onions. Once sizzling, reduce the heat to low and cook gently, stirring from time to time, until they are soft and golden, about 10-15 minutes, adding the garlic halfway through.
Shred the kale into 1/2-1cm wide ribbons. Stir them into the onions and cook for a further 5 minutes, stirring often, until the leaves have wilted. Season with salt and pepper.
Roll out your pizza dough as thinly as you can (if you have made your own).
Scatter a peel or another baking sheet with a little flour and place the dough base on it. Spread a third of the kale and onions on the pizza base, then top with a third of the grated Cheddar.
Slide the pizza onto the hot baking sheet in the oven (for a really crispy crust) or you can simply lay the baking sheet on the hot one in the oven (to avoid the tricky pizza transfer). Trickle with a little more oil and bake for 10-12 minutes. Repeat with the remaining topping and cheese, and serve, cut into wedges.