Peel both honeynut squash, cut in half, and remove seeds with a spoon. Slice the squash 1/4 inch thick with a knife or mandolin slicer.
Cut off the green tops off the leeks and the end. Cut in half and wash well. Then slice thin.
Into a saute pan, melt 2 tbsp of butter with 2 tbsp of olive oil. Add the leeks to the pan and sprinkle with salt. Saute the leeks until soft. Set aside.
Preheat the oven to 425℉.
Into a 9x9 baking dish or an oval dish, take a tbsp of butter to grease the dish.
Add half of the squash, sprinkle with salt and pepper. Top with cooked leeks and half the gruyere cheese. Add the rest of the squash and season with salt and pepper. Top with the rest of the Gruyere cheese and grated parmigiano.
Cover with a piece of parchment paper and then a piece of aluminum foil. Bake for 30 mins covered.
While it's baking, melt 2 tbsp of unsalted butter. Add the butter to 1/2 cup of Panko breadcrumbs and mix well.
After 30 mins, remove the cover and top with buttered breadcrumbs. Place back with the oven to bake uncovered for another 30 mins, until golden brown on top and bubbling.
This can be made a day ahead and gently reheated in the oven at 325℉ until warmed through.