For the marinade: Combine balsamic vinegar and runny honey. Dilute with olive oil, salt and pepper.
Peel the beets, cut in half and cut into slices 4-5 mm thick. Mix marinade with beets, chard and spinach, leave for 10 minutes.
Toast the walnuts in a dry frying pan for 2-3 minutes. Cool and chop. Finely chop the goat cheese.
Line a small heat-resistant tin with a large piece of foil, leaving loose ends on all sides of the tin. Lay the beetroot overlapping, alternating with the cheese. Sprinkle the layers with walnuts.
Bring the edges of the foil together, covering the beets. Bake at 180°C for 45-50 minutes. Remove from oven, open foil, sprinkle with chopped cilantro and drizzle with lemon juice. Close again and let the beets cool to room temperature.