For the salad: Preheat the oven to 400 degrees F.
Cook the beets in a saucepan of salted boiling water until soft, then drain, peel and quarter. (If you like, use kitchen gloves for the peeling part to avoid staining your hands pink.)
On a baking sheet, toss the quartered beets with the olive oil and then the coriander and cumin until coated. Roast on the middle shelf for 15 minutes.
Add the baby spinach to a serving platter and sprinkle over the lentils. Add the roasted beets and the red onions, scatter over the mint and parsley, and crumble the feta over the top.
For the dressing: Mix together the olive oil, balsamic vinegar, honey and some salt in a small saucepan over medium heat. Heat just until warm. Adjust the honey and salt to taste.
Drizzle the dressing over the salad just before serving.