Melt the butter or coconut oil in a skillet over medium heat, and saute the onion for 5 minutes. Add the garlic and saute until fragrant, about 1 minute. Add the chopped squash into the pan, season with salt and red pepper flakes, and saute until the squash is as tender as you'd like it to be, about 10 to 15 minutes. Stir in the yogurt, then sprinkle a layer of shredded cheddar over the top.
Set the oven's broiler to high, then place the pan 6 inches under the heat source to broil until the cheese is bubbly, about 3 minutes. Use a slotted spoon to serve warm, and enjoy!
Depending on the location of your oven's broiler, you may need to alter the size of the pan you use. I originally started this dish in a 3.5 qt. Dutch oven, but it doesn't fit under my gas oven's broiler, so I switched to using a cast-iron skillet before broiling.