Preheat the oven to 400°F or 200°C. Line a baking sheet with parchment paper.Cut the squash in half and scoop out its seeds.Put it cut side down on a cutting board, then cut off the peel with a sharp knife.
Chop into chunks roughly the same size, transfer to the baking sheet and toss with olive oil and salt.Roast for 30 minutes until fork tender and slightly charred.
Transfer roasted kabocha squash to a food processor and add the ricotta cheese, parmesancheese, salt, pepper, and water.
Blend until you have a smooth and creamy sauce, then transfer it to a large skillet.You should get consistency between a squash puree and a soup.
Boil the pasta al dente in a large pot with plenty of salted boiling water.Reserve one cup of pasta water, drain the pasta and add it to the pan with the sauce.Toss on medium heat for a minute, adding ½ to 1 cup of the reserved pasta water.
Transfer kabocha squash pasta onto a serving plate and sprinkle withpistachio nuts, thyme leaves, and freshly ground black pepper.