Preheat oven to 425 degrees Fahrenheit (200 degrees Celsius).

Rub the beets in olive oil and wrap each one separately in aluminum foil. Bake for 50–60 minutes (less for smaller beets). Unwrap. When the beets are cool enough to handle, rub off the skins, dice, and set aside.

Heat one tablespoon olive oil in a large skillet over medium heat. Add onions and cumin and sauté until translucent. Add the couscous, salt, and pepper, and stir, coating the couscous pieces in the oil. Add boiling water and turn off flame, cover, and let stand for 15 minutes. Fluff with a fork and transfer to a large serving bowl.

Add the reserved beets, lemon juice, cucumber, mint, parsley, and remaining one tablespoon of oil. When ready to serve, toss with the spinach leaves. Serve at room temperature.