Start by stirring the mascarpone. Add the grated rind of 1 lime and some juice. It is also nice to add a tablespoon of the stewed pear juice.
It's not sweet, but since the caramel sauce will be on top, you won't need it.
Make a bed of the mascarpone on a serving platter and place the poached pears on top. Sprinkle with chopped walnuts.
Orange caramel sauce
Place the sugar in a saucepan and add three tablespoons of water. Make sure all the sugar is wet.
Place the saucepan over medium heat and bring the sugar to a boil. Do NOT stir the pan. Above all, stay with it, because once it starts to brown, it will go in a flash.
If the caramel gets browner on one side than the other, gently swirl the pan back and forth to even it out.
When the caramel is the right color, add the orange juice. Be careful, it will bubble a lot. It may also suddenly form a lump. Just keep stirring gently and it should dissolve.
Now add the butter, chunk by chunk, until you have a nice caramel sauce. At this point it will still be thin, but as the caramel cools it will thicken. Pour the cooled sauce over the poached pears and serve.