MAKE THE GNOCCHI: Steam the squash and potatoes in a large pot fitted with a basket steamer, cooking for 12 minutes until the pieces can be pierced easily with a knife. Remove the steaming tray from the pan, and let drain and cool for 10 minutes. Pass the potatoes and squash through a potato ricer or food mill fitted with a middle-holed disc into a large bowl. Drizzle with grappa, if using. Add the flour and squeeze the dough in your hands until the flour is absorbed. Lightly dust your work surface with flour and transfer the dough to it. Knead the dough until it begins to stick. Transfer to the bowl and cover with plastic wrap. Let stand for 15 minutes. Dust 2 trays that will fit in your freezer with flour. Divide the dough into 8 pieces. Dust your work surface with 1 tablespoon of flour. Working with 1 piece of dough at a time, gently press and roll out the dough with your hands to form a long rope that is 18 inches long and 1⁄2 inch in diameter. The strip of dough should be covered with flour. Cut into 1-inch-long pieces. Turn a fork over, and roll each piece from the top of the arch to the tip of the fork, pressing with your index finger to make an indentation on the underside of the gnocchi. Place the gnocchi in the freezer, making sure to keep them apart so they do not stick. Freeze them uncovered on the tray until cooking, or transfer them to a sealed plastic bag once they have hardened and freeze for up to 1 month. MAKE THE SAUCE: Combine the tomatoes, onion, oil, and salt in a medium skillet. Simmer on medium-low heat for 20 minutes uncovered. Turn off the heat; remove the pieces of onion and stir in the basil leaves. In a large pot, bring 3 quarts of water to a rolling boil and add the salt. Lift up the gnocchi from the tray with a spatula and drop into the pot. Bring back to a full boil. Cook for 3 to 4 minutes until the gnocchi are soft but still have some bite in the center. Drain and serve right away, topped with the tomato sauce and grated cheese.