Preheat the oven to 375 degrees F. Toss the pears, nutmeg and half of the lemon juice together in a bowl.
Melt the butter with the remaining lemon juice, the salt and 3 tablespoons water in a heavy ovenproof 10-inch skillet over medium-high heat. Sprinkle the sugar into the skillet and stir to combine. Cook the mixture without stirring, letting it foam and bubble; the butter will separate, which is OK. Watch the mixture carefully while it cooks because once the water evaporates and the sugar begins to change color, things happen quickly. When it becomes the color of light brown sugar, about 10 minutes, remove it from the heat. Stir in the vanilla.
Being careful not to burn yourself, sprinkle the cranberries and ginger into the caramel. Arrange the pears in the skillet round side-down in concentric circles as nicely as you can, fitting in as many pears as possible.
Return the skillet to the burner over medium heat and cook until the pears start to soften when you poke them with a knife, 10 to 15 minutes.
While your pears cook, roll out your Pie Crust on a flour-dusted surface to an 11-inch circle. Cover the crust with plastic wrap and refrigerate until ready to use.
Lay the crust over the pears, tucking it in around the edge of the skillet with a wooden spoon. Bake until the crust is golden brown and firm to the touch, about 30 minutes.
Allow the tart to cool until you can handle it, about 10 minutes. Put a plate on top of the tart using heatproof pads and flip it over so the pears are on top. Slice and serve warm with creme fraiche, if using.
Cook's Notes: This tart tastes great served warm but if you decide to serve it chilled, turn it out and chill it without the pan. The tart will release lots of liquid, but I found that later it will re-absorb it.
Stop cooking the caramel when it turns the color of light tea. Once it begins to change color, that means it's cooking fast. If you want it to stop the cooking, take it off the stove and put a big metal spoon in it.
Combine the flour, sugar, salt and butter in a food processor and pulse until the butter is evenly distributed but still visible in chunks. While pulsing, add the ice water 1 tablespoon at a time until the dough is just wet enough to hold together when pinched between your fingers. Turn the dough out onto a piece of plastic wrap, form into a disc and refrigerate for 30 minutes or up to 24 hours.