Set heavy 12-inch skillet that has a lid (which you'll use later) over medium heat. Pour in hazelnuts. Cook, shaking the pan occasionally, until skins have turned dark brown in spots and begun to split, about 15 minutes.
While hot, carefully pour hazelnuts into center of clean dishtowel. Rub together to remove skins. When hazelnuts are cool enough to handle, finely chop and set aside.
Wipe out skillet to remove any hazelnut bits. Set over medium-high heat. Add butter and cook until bubbly and beginning to turn brown.
Reduce heat to medium. Add cauliflower, sage, chopped hazelnuts, salt, and pepper and cook, stirring occasionally, until cauliflower has caramelized in spots, about 10 minutes.
Pour in two tablespoons water, cover pan, and reduce heat to low. Continue cooking for a few more minutes, until cauliflower is as tender as you'd like. Uncover pan and simmer off any remaining liquid, then remove from heat.
Quarter and core pear and slice thinly. Add pear, along with parsley, if using, to cauliflower mixture and stir to distribute. Serve warm.