Put the frozen kabocha squash on a heatproof dish and microwave without a cover. Important: cook it for a bit longer than indicated on the frozen packet, to make it less watery and denser in flavor. I microwaved it for 8 minutes at 600 W.
Edited: Keep you eye on the kabocha while you're cooking it and cover with plastic wrap if the kabocha is getting too dry.
If it still feels water after you push a piece in with your finger, microwave it for a bit more, until the kabocha squash is cooked through. It should be creamy yet floury.
If you cook it for too long, it will turn yellow as shown here. It'll be dry and grainy and not very nice, so be careful.
While the kaboccha squash is cooking, cut the cheese up.
Cut up the cooked kabocha squash into easy to eat pieces. (By cutting it up you'll get rid of more excess moisture.) Put the cut up cheese on top; it will soften with the residual heat. Leave to rest for a while until cool.
Chop up the egg coarsely. The flavor will become milder with the egg.
When the kabocha squash is cool, add the egg and mayonnaise, mix, and it's done. Taste, and add more mayonnaise and/or salt and pepper as needed.
Story Behind this Recipe
I experimented a lot before coming up with this recipe. My mother-in-law, who's a great cook, always praises this salad too! My family likes rice and dense flavors so I use 3 cheeses and mayonnaise in it.
Use the recommended cheese, and be sure to get rid of any excess moisture. Please adjust the cooking time depending on your microwave. If you need to heat it for longer, don't forget to cover it with plastic wrap to prevent it from drying out.