To poach the pears: In a large pot, combine the water and sugar over medium-high heat. Using a vegetable peeler, cut wide strips of lemon zest from the lemon and add to the pot along with the juice of the whole lemon. Bring the mixture to a boil, stirring to dissolve the sugar, then reduce the heat to maintain a gentle simmer. Submerge pears in the simmering liquid, adding more water, if necessary, to just cover the pears. Cut a piece of parchment paper into a circle just large enough to cover the pot. Ease the paper down into the pot until it is touching the surface of the liquid. Simmer the pears until a knife inserted directly into a pear meets little resistance, about 30 minutes. Remove the pot from the heat and cool the pears completely in the poaching liquid.
To make the sauce: In a saucepan, over high heat, combine the brown sugar and water and bring it to a boil. Reduce heat to medium and simmer until the sauce is reduced, thicker and takes on a dark golden brown color, about 8 to 10 minutes. Carefully stir in cream, reduce the heat to medium-low and simmer until very thick, 2 to 3 minutes more. Stir in the toasted pecans and salt.
Serve 1 to 2 pear halves per person, drizzled with the creamy pecan caramel sauce.