Halve and peel onions. Slice halves into thin strips. Cut off 1/4" from top and bottom of blood orange. Stand orange on end and carefully cut off peel, following the curvature of fruit. Slice orange into 1/4" rounds and cut rounds into 1/4" dice.

Heat 2 tsp. olive oil in a large oven-safe non-stick pan over medium heat. Add onions to hot pan and cook undisturbed until lightly browned, 4-5 minutes. Season with 1/4 tsp. salt and a pinch of pepper. Continue cooking, stirring occasionally, until caramelized, 6-8 minutes. Add half the balsamic vinegar (reserve remaining for salad) and stir constantly until nearly evaporated and onions are coated, 2-3 minutes. Transfer to a plate or bowl. Wipe pan clean and reserve.

Place a medium non-stick pan over medium-high heat. Add sugar and 2 Tbsp. water. Bring to a simmer and swirl until sugar is dissolved. Add pecans and cook, stirring constantly, until water has evaporated and pecans are coated in a sticky glaze, 3-5 minutes. Transfer to a plate. Hot nut alert! Be careful when transferring.

Top each bread slice with Swiss cheese and caramelized onions. Close sandwiches. Return pan used to cook onions to medium heat. Add butter and sandwiches to hot pan. Work in batches if needed, using half the butter for each sandwich. Cook until browned on one side, 2-3 minutes. Flip and transfer pan to oven. Bake until well-browned and cheese is melted, 3-4 minutes. Rest 3-5 minutes before slicing. While grilled cheese bakes, toss salad.

In a medium mixing bowl, whisk together remaining balsamic vinegar, 1 Tbsp. olive oil, 1/4 tsp. salt, and a pinch of pepper until well-combined. Add arugula and blood orange and toss to coat. Taste, and season with a pinch of salt and pepper if desired.

Serve grilled cheese on a plate along with salad. Garnish salad with candied pecans.