Heat a large skillet over medium-high heat. Add olive oil, butternut squash, leek, salt, black pepper, and garlic powder.
Cook for about 15 minutes, stirring periodically. The butternut squash should be mostly cooked at this point.
Add the ham and cook for 5 more minutes. Check that the butternut squash is fork-tender.
Mix in the cooked rice and let the mixture cool so that it is no longer steaming hot. You can cool the mixture in the fridge for about 10 minutes.
Preheat the oven to 350 degrees and prepare a muffin tin with non-stick cooking spray.
Whisk eggs, milk, salt, and black pepper. Pour the eggs and feta into the cooled squash mixture and mix.
Spoon into muffin tins, you should be able to make 12. Bake for 20-25 minutes until the eggs are set.
Enjoy immediately or let cool and store in the refrigerator for breakfast throughout the week. To reheat, microwave for 1 minute.