Step 1For the crust, combine the flour, salt and pepper in a mixing bowl and toss with a fork to mix well. Cut the chilled unsalted butter into one-quarter-inch pieces and rub into the dry ingredients with fingertips until the mixture resembles very coarse meal. Sprinkle with 2 tablespoons ice water and toss until the ingredients cling together, adding 1 tablespoon more water if necessary. Pull together into a ball and knead very lightly, then pat out into a thick round on wax paper. Wrap the dough in the wax paper and chill it while cooking the pumpkin.
Step 2Heat the oven to 375 degrees. Peel and seed the pumpkin and cut it into one-quarter-inch-thick slices.
Step 3Combine the olive oil and 2 tablespoons of the butter in a 10-inch (measured across the top) nonstick, ovenproof skillet over medium-low heat. Add the onion, salt to taste and half the thyme and saute, stirring often, until very soft and caramelized, about 20 to 25 minutes. Transfer to a bowl.
Step 4Wipe the skillet clean and add the remaining butter and the pumpkin seed oil; melt over medium heat. Arrange the pumpkin slices in the skillet in slightly overlapping layers, but with most of the pumpkin flat on the skillet so the surfaces will caramelize. The pumpkin should cover the bottom completely. Sprinkle with the remaining thyme and season with salt and pepper. Cook over medium-high heat until the bottom slices start to caramelize, about 5 minutes. Reduce the heat to the lowest setting, cover the skillet and cook until the pumpkin is soft but not falling apart, about 10 minutes. Drizzle with the cream and remove from the heat. Crumble the goat cheese and scatter the onions evenly over the pumpkin.
Step 5Cut a sheet of wax paper into a 10-inch round. On a lightly floured surface, roll out the dough under the sheet to make a crust, using the sheet as a guide. Remove the wax paper and carefully fit the crust over the pumpkin, tucking and crimping the perimeter to seal it completely.
Step 6Bake in the top third of the oven for 30 to 35 minutes, until the crust is browned. Using a small spatula around the edges of the skillet to release the crust, immediately unmold the tarte onto a serving platter (place a platter over the skillet and invert it). Cut it into wedges and serve warm or hot.