Heat your oven to 375F. Toss the squash cubes with the olive oil and some salt and freshly ground pepper. Pour them onto a roasting pan or baking sheet, in a single layer, and bake for 25 minutes (or until soft with a little bite left.) Toss them at least once so they don’t burn. Set aside to cool.
Meanwhile, add all dry ingredients (flour, oats, baking powder, nutmeg and cinnamon) into a mixing bowl and combine. In another bowl, whisk together the milk, eggs, sugar and salt. Prepare a 12-cup muffin pan by greasing or lining the cups.
Once the squash is cool enough, add in the grated cheese and leek, and stir to combine. Pour over your liquid ingredients, and then dump in the dry ingredients and mix with a wooden spoon until just combined.
Spoon large dollops of the batter into the muffin pan, overflowing each cup slightly. Bake for 20 minutes, until the muffins are golden and the toothpick test works (insert one into the center and if it comes out clean, it’s done!)
Remove from the oven, allow to cool for about 5 minutes, then slide a knife around the circumference of the muffin to pop it out and cool on a rack.