Lightly grease a 9-inch tart pan with removeable sides.
In a food processor, combine the flour, sugar and salt and pulse briefly to mix.
Add the shortening and cut it into the flour mixture using quick on/off pulses until the mixture resembles coarse meal. Add the egg and the vanilla and process just until moist clumps form, pulsing the processor on and off.
Gather the dough into a ball and using your fingertips, press evenly into the bottom and up the sides of the preapred tart pan. Refrigerate until ready to use.
Halve each pear lengthwise and scoop out the core with a melon baller or spoon. Peel and cut crosswise into 1/4-inch slices. There should be about 5 cups.
Melt the butter over medium heat in a large non-stick skillet and stir in the brown sugar. When the sugar has dissolved and is bubbly, add the pears to the skillet sprinkling them with lemon juice and let the pears cook in the sugar syrup, stirring gently so as not to break up the slices. The pears will, at first, release quite a bit of juice - allow this to simmer until the liquid reduces to a thick syrup and the pears have begun to caramelize. Remove from heat and let cool completely.
When ready to bake, preheat the oven to 425°F Gently spoon the pears into the prepared tart shell, spreading them out evenly. Sprnikle with the sliced almonds. Bake for 25 minutes, then reduce the heat to 350F and continue baking for 25-30 minutes, until the almonds are toasted and pear filling is bubbly.
Let cool before removing the sides of the tart pan to serve.