Make the dressing: In a small saucepan over medium-low to low heat, reduce the balsamic vinegar to 1/3 of the original volume, swirling regularly to prevent burning. Depending on the heat level, this will take up to 25 minutes.
Once vinegar is reduced, remove from heat and add the orange juice and cinnamon, whisking to combine. Set aside to cool.
Prepare the salad: Separate the leaves of the radicchio and tear them into bite-size pieces. Discard any bulky sections that are exclusively white center vein. Wash and dry.
Slice the fennel in half and cut a triangle to remove most of the tough root base. Slice the blub into very thin (less than ¼”/6mm) sections.
Wash and dry pears. Cut each pear into 8-10 pieces lengthwise and remove the stem and core.
Add the chopped walnuts to a large pan over medium heat. Toast the nuts until fragrant and slightly browned, stirring frequently. Once toasted, remove nuts to a bowl to cool.
Return the pan to the heat and add the olive oil. Once hot, add in the brown sugar and stir until melted.
Carefully add the pear slices so one cut side is down. Cook for 3-5 minutes until beginning to brown. Flip and cook the other side.
Once both sides of the pear are golden brown and slightly softened (but still holding their shape), remove from the pan to drain.
Assemble the salad in a large bowl or individually on plates. Toss the radicchio with the sliced fennel and pears. Drizzle on the dressing and toasted walnuts. Add freshly ground black pepper and flaky sea salt to taste.
If assembling individually, place 3-4 pear slices per serving. Serve immediately.