Peel and finely chop the garlic. Wash the lemons hot, rub dry, finely grate the peel and squeeze the juice. Mix cumin, coriander, chilli flakes, garlic, sugar, 1 teaspoon salt, 1 tablespoon lemon juice, lemon peel and oil to make a spice oil. Peel sweet potatoes and cut them into cubes of about 3 cm. Mix thoroughly in a bowl with the spice oil. Place the sweet potatoes on a baking tray lined with baking paper and cook in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for 20-30 minutes.
Halve the avocado, stone it and remove the flesh from the skin. Cut into large pieces and sprinkle with 1 tbsp. lemon juice. Clean romaine lettuce, cut into strips, wash and spin dry. Wash mint, shake dry, pluck leaves from the stems and cut into strips.
For the dressing, mix Tahini, 2 tablespoons lemon juice, 50-60 ml water and honey, season with salt and pepper. Remove the tray from the oven, let the sweet potatoes cool lukewarm. Mix the salad ingredients in a large bowl, drizzle with the dressing and serve immediately.