Make the Beet and Celery Root Cakes:  Using the large holes on a box grater or the grater function of your food processor, grate beets, celery root and onion.

Crack eggs into a large bowl and beat lightly.  Add grated vegetables, flour, cumin, coriander, salt and a few grinds of pepper.  Stir to combine.

Heat 3 tablespoons of olive oil in a large nonstick skillet over medium heat until shimmering.  Add ¼ cup of mixture to the pan at a time, pressing down slightly with the back of the measuring cup to flatten into ½-¾ inch thick cakes. You should be able to fit 6 cakes in the skillet at a time.  Cook until golden brown, crisp and cooked through, about 8 minutes total, flipping cakes occasionally. Lower the heat as necessary if the cakes are browning too quickly.

Transfer to a paper towel lined plate.  Sprinkle with sea salt flakes.

Wipe out skillet and add the remaining 3 tablespoons of olive oil.  Repeat the process with the remaining mixture.

Assemble the Smørrebrød:  Transfer bread slices to a work surface.  Spread with butter.

Place a small handful of arugula on each slice of bread.

Top arugula with 2 cakes per sandwich.

Artfully dollop a spoonful of Herbed Skyr Sauce on top.

Garnish with dill or chives, if desired.  Serve.