Place cauliflower florets in a steamer basket set over a pot of simmering water. Cover and steam until crisp-tender, about 8 minutes. Add spinach leaves; cover and steam until wilted, about 2 minutes. Puree vegetables in a food processor with tahini, lemon juice, olive oil, garlic, salt, and pepper until smooth. Serve warm or at room temperature with pita bread or crisps.