Preheat oven to 200°C.
Wash beetroot and trim stems and roots, reserving any small beetroot leaves.
Wrap beets in a large sheet of foil and bake for 1 hour, or until the largest beetroot is tender when tested with a fork.
Combine olive oil, vinegar, mustard and garlic in a small screw-top jar. Shake to combine. Leave to stand for at least 15 minutes. Remove and discard garlic. Carefully unwrap beetroot. Leave until cool enough to handle. Peel off skin (wear rubber gloves to avoid staining your hands). Cut into quarters, or halves for the smaller ones.
Combine spinach leaves, reserved beetroot leaves, chickpeas and onion in a large bowl. Add dressing and toss to combine.
Divide between serving plates, top with beetroot, then crumble fetta over the top. Sprinkle with freshly ground black pepper and serve immediately.