Preheat the oven - Preheat the oven to 200°C (400°F) and line a baking sheet with parchment paper.

Prepare the squash - Peel the kabocha squash, remove the seeds, and cut it into bite-sized cubes. Place the cubes on the prepared baking sheet, drizzle with olive oil, sprinkle with salt and pepper, and toss to coat. Roast in the preheated oven for 20-25 minutes, or until the squash is tender and slightly caramelized. - If you find it challenging to peel and cut the squash, you can also cook it whole in the oven until tender, then peel and cut it.

Prepare the puff pastry - Roll out the puff pastry sheet on a lightly floured surface to a rectangular shape. Using a sharp knife, cut the pastry into 4 squares or circles, depending on your preferred shape and size of tartelettes.

Assemble the tartelettes - Place a generous spoonful of roasted kabocha squash in the center of each puff pastry square or circle. Crumble feta cheese over the squash and sprinkle with fresh rosemary leaves. Fold the edges of the puff pastry slightly inward to create a rustic crust. - For an extra touch, you can brush the edges of the pastry with beaten egg to give it a golden and shiny appearance.

Bake the tartelettes - Transfer the assembled tartelettes onto a baking sheet lined with parchment paper. Bake in the preheated oven for 10-15 minutes, or until the puff pastry is golden brown and crispy. - Keep a close eye on the tartelettes in the oven as cooking times may vary. You want the puff pastry to be fully cooked and slightly puffed.

Serve and enjoy - Allow the tartelettes to cool slightly before serving. These savory kabocha and Feta Tartelettes with Rosemary are best enjoyed fresh out of the oven. Serve as an appetizer or as part of a delicious meal and savor the unique combination of flavors!