Very lightly oil one of the cut sides of the squash and season with salt. Place the potato and squash on a sheet pan and bake until both are tender and able to be pierced with a knife, 30 to 45 minutes.
Scoop the squash out of the outer skin, transfer to a blender and blend until smooth. Set aside.
When the potato is just warm and there is no visible steam, add the butternut squash puree, flour, cheese, egg yolk and some salt. Using your hands, mix to form a dough. Once the dough forms, move to a smooth surface and gently knead the dough for 3 to 5 minutes, adding flour as needed. Cover and allow the dough to rest for 15 to 20 minutes.
Cut the dough into strips and roll to form the cut dough into even shaped logs. Cut the gnocchi to size, each about 1-inch long. Use a fork on a floured surface to shape the gnocchi into uniform pillows
Put the gnocchi in boiling salted water just until the gnocchi float, only a few minutes.
For the sauce: Melt the butter in a small pot over medium-low heat, then add the shallots and leeks and sweat without achieving any color, 5 minutes. Add the wine and reduce for 2 to 3 minutes. Add the stock, lower the heat and simmer for 5 minutes. Add the cream and cook for 3 to 5 minutes. Transfer the sauce to a blender, process until smooth and adjust the seasoning.
For the swordfish: Heat two pans over medium-high heat, one with the butter and one with the oil.
Sprinkle the fish with salt and pepper, and sear in the oiled pan. Once it is seared, flip the fish and lower the heat until cooked through.
To serve, arrange the gnocchi on a plate or bowl. Place the fish on top and spoon the leek sauce around. Garnish with crispy fried leeks, if desired.