Kabocha is called Japanese pumpkin because of its color, but the flavor is a bit different than its American counterpart (and Japanese people eat the skin) and I've never heard of it being used in a pie. It's usually served as a bento lunchbox side dish, with a meal as a savory or sweetened veggie, or in a boiled meal (nimono) with some kind of meat. I served mine with okonomiyaki, a kind of noodle pancake, so stay tuned for that recipe another day.
For this simple recipe that I got from my co-op catalog, you just cut up 1/4 kabocha (or about 1 lb. pumpkin if you have it) into bite-sized pieces, throw it in a microwaveable container with a lid, nuke it for two minutes, take it out, add 1/2 t. mirin (a sweet Japanese cooking liquid, or you could just add some sugar) and 1/2 t. dark soy sauce, stir it all around, nuke it for another two minutes, or until soft, take it out, and toss with 1 T. ground sesame seeds. You can buy them whole or ground here in Japan, but I paid all of 100 yen (about $1) for my mortar and pestle so I do like to get value for money out of it. Plus it gives my arm muscles a good workout.
Here's the toddler-bean eating the sesame seeds ONE AT A TIME. No joke.