The night before, heat oven to 200C/180C fan/gas 6. Put the cauliflower and broccoli in a large roasting tin with the oil and a sprinkle of flaky sea salt. Roast for 25-30 mins until browned and cooked. Leave to cool completely.
Build each bowl by putting half the quinoa in each. Lay the slices of beetroot on top, followed by the spinach, cauliflower, broccoli and walnuts. Combine the tahini, houmous, lemon juice and 1 tbsp water in a small pot. Before eating, coat in the dressing. Serve with the lemon wedges.