In the work bowl of a food processor, pulse together fontina cheese, pine nuts, and mustard until combined and smooth.
Spread an even layer of pine nut spread on 6 bread slices. Set aside.
Divide arugula among remaining 6 bread slices. Layer 2 ham slices over arugula on each bread slice. Top each with a reserved bread slice, pine nut spread side down.
Using a serrated knife, in a gentle sawing motion, trim and discard crusts from sandwiches. Cut each sandwich diagonally in half, creating 2 triangular sandwiches. Cover with damp paper towels, place in a covered container, and refrigerate until serving time.
Just before serving, garnish with arugula and toasted pine nuts, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/ham-tea-sandwiches-with-pine-nut-spread-recipe/