Roast the squash. Peel and deseed a medium butternut squash (about 2 lb or 900 g raw), then cut it into 1-inch (2.5 cm) cubes. Spread the cubes on a rimmed baking sheet and roast at 400°F (200°C) for 25 to 35 minutes, until completely tender. Let the squash cool slightly. Then measure out 25 oz (700 g) of cooked squash, which is about 2 ¾ cups, if you want to follow the quantities precisely.
Blend the squash layer. Add the cooked squash, 1 large egg, 3 level tablespoons (25 g) cornstarch, the zest of ½ orange, 3 oz (85 g) sugar and just ¼ cup (60 ml) of orange juice to a clean blender. Blend until smooth and add a splash more juice only if the mixture is too thick to blend. Because squash can be watery, do not add the full ⅓ cup (80 ml) unless you really need it.
Blend the cottage cheese layer. Add cottage cheese, 2 large eggs, Greek yogurt, 3 level tablespoons (25 g) cornstarch and 3 oz (85 g) sugar, which is a scant ½ cup, to a blender. Blend until perfectly smooth. Taste the mixture and adjust the sweetness if you like.
Layer and swirl. Lightly grease an 8-inch square or 9-inch round baking dish, about 2 inches (5 cm) deep, which is roughly 20 to 22 cm. Alternate spoonfuls of the cottage cheese mixture and the squash mixture across the dish. Then drag a knife or skewer through the batter once or twice for a marbled effect, or leave the layers as they are for a bolder contrast.
Bake. Bake at 350°F (180°C) for 50 to 60 minutes, until the casserole is set, lightly golden on top and golden around the edges. If the center is still very liquid, keep baking in 5-minute increments, checking each time.
