Cut the cooked, cooled beets into wedges or bite-sized pieces. Place in a mixing bowl with the red onion.
Whisk the lemon juice, Dijon mustard, honey, garlic, and salt in a small bowl. Slowly whisk in the olive oil until combined.
Pour the dressing over the beets and onion. Toss gently, cover, and refrigerate for at least 30 minutes.
Add the dill and feta just before serving. Taste and adjust the salt or lemon juice as needed.
Notes
Beets: Use one pound of cooked, peeled beets. To cook fresh beets, steam whole beets for 35 to 55 minutes, until tender. Cool and peel before assembling. Make ahead: Dress the beets and onion up to a day ahead. Add feta and dill before serving.Storage: Refrigerate in an airtight container for up to 3 days.