Pulse flour, cornmeal, salt, and sugar together in the bowl of a food processor. Add the butter and pulse at 5-second bursts until mostly pea-size pieces of butter remain (a few larger pieces are okay; be careful not to overblend). Combine the water, cider vinegar, and ice in a measuring cup or small bowl. Drizzle 2 tablespoons of ice water mixture and pulse until incorporated. Add more of the ice water mixture, 1 to 2 tablespoons at a time, and pulse until the dough comes together, with some dry bits remaining. Squeeze a clump of dough between your fingers - if it holds together, it is ready. If it is still fairly crumbly, add a little more ice water-vinegar and pulse again until just combined. Shape dough into a flat disc, wrap in plastic, and refrigerate for at least 1 hour to give the pastry time to mellow. Wrapped tightly, dough can be refrigerated for 3 days or frozen for 1 month.

Bring pastry out of fridge and remove plastic. On a lightly floured work surface, roll it to 1/4-inch thick, turning slightly with each go of the rolling pin for evenness. Drape dough into an 8-inch springform pan, freeing it from the edge as you gently press it to the base of the pan with your fingertips. Trim any particularly high points or thick edges to patch thinner/shorter areas or mend cracks. Dock dough all over with a fork and chill in the refrigerator.

Sauté onion over medium-low heat in a cast iron skillet in a good glug or two of olive oil. Stirring occasionally, do not brown the onions: sweat them, rendering them translucent, then sauté towards caramelization. This is a 20 minute or so process where you'll occasionally stir them, and then return to handling the remainder of the recipe. If you don't have that time, many groceries carry onion jam or a package of caramelized onions in the specialty department.

Meanwhile roast delicata (or other squash), tossed in a tablespoon or two of olive oil and seasoned with sea salt and freshly cracked pepper, on the stovetop or in the oven. For the oven, roast for 15-20 minutes at 400° until the bottom sides are deeply golden but the flesh is still soft, or in a cast iron skillet over medium heat for the same result. This can be done a day or two in advance and refrigerated until needed.

If you did the above in advance, preheat oven to 400°. Place a sheet of parchment over dough and weight with baking weights, beans, or rice. Blind bake for 15 minutes or according to your oven. Check and see that the dough is not wet or particularly soft. Remove weights and paper and bake for a further 10 minutes or until crust is golden. Remove from oven and lower temperature to 300°.

Whisk together eggs, cream, nutmeg, cayenne, thyme, garlic chives, salt, and pepper in a large bowl. Add the squash and onions and pour mixture into pastry. Top with a few grinds of pepper and bake for 30-40 minutes, until custard has *just* set. There should be a slight jiggle in the center when agitated. Cool inside the oven with the door ajar for 10 minutes or so, and then cool fully on a wire rack.

Release springform collar from pastry. Use two spatulas or palette knives to transfer quiche to a serving platter. Serve with a lemony-dressed green salad and enjoy!