Preheat oven to 400°F.
Wrap beets in parchment and foil, and roast until tender, for about 1 to 1 ½ hours. Unwrap beets and allow to cool.
While beets are roasting, slice the onions, crumble the feta and prepare the vinaigrette.
When the beets have cooled, peel with a paring knife and cut into cubes. Place in a bowl with the sliced onions and toss with 2 tbsp of vinaigrette.
Arrange beets on a platter over the arugula and sprinkle with the crumbled feta. Drizzle with more vinaigrette and top with micro-greens.