Preheat oven to 200 C / Gas 6. Coat a baking dish with olive oil.
Place pumpkin in a steamer over 2.5cm of boiling water, and cover. Cook until tender but still firm. Transfer pumpkin to baking dish, and brush with oil. Season with salt and pepper. Bake in preheated oven until golden brown.
Meanwhile, heat olive oil in a saucepan. Cook onion and garlic until tender. Stir in rice, and cook for 1 to 2 minutes. Slowly add vegetable stock 125ml at a time, stirring frequently and allowing all the liquid to be absorbed before adding more stock. Continue cooking until the rice is tender; season with salt and pepper to taste.
When the pumpkin is almost done, place feta on a baking tray, and bake in preheated oven until the cheese is hot, and is beginning to melt along the edges.
Mash half of the roasted pumpkin cubes; stir into the risotto with the spinach. Spoon risotto on to plates; top with remaining pumpkin cubes and heated feta.