In a medium bowl, whisk together all ingredients for the dressing. Slowly whisk until the olive oil has emulsified. Set aside and let stand for at least 20 minutes to allow the flavors to marry.
Using a paring knife cleaning remove the rind and divide slices of both the navel and blood oranges. Then slice the cleaned and trimmed fennel bulb. (Tip: Using a mandolin makes this a breeze!). In a medium bowl, combine fennel, feta, and orange slices.
On a large flat dish, arrange a bed of arugula and transfer the oranges, fennel and feta evenly across.
Lightly drizzle with dressing, reserving extra for guests to add to taste.