Preheat oven to 180 °C. Cut off the long necks of the squashes. Put hollowed-out round parts on a baking tray and pour olive oil over. Bake for 20 minutes until soft. Meanwhile, remove skin of the long parts and cut flesh into small cubes. Heat some more olive oil in a pan and fry cubes until soft – about 10 minutes. Add spinach and stir carefully until wilted. Season with salt and black pepper. Remove from heat and add lemon juice and feta. Take squashes out of oven and stuff with the filling. Put under the grill until cheese has melted.