1. Preheat the oven to 240 ̊C, gas mark 8. Toss the sweet potato with 1 tsp oil and the garlic; season and roast for 30 minutes.
2. Meanwhile, heat a griddle pan over a high heat. Rub 1 tsp oil over the corn, season and griddle for 15 minutes, turning regularly; set aside. Toss the avocado in a bowl with the remaining 1 tsp oil and the juice of 1⁄2 lime. Season and griddle for 2 minutes on each side; set aside.
3. Whisk the soured cream, lime zest, tarragon and chipotle in a bowl; season. On a platter, pile up the rocket, herbs and sweet potato. Slice the corn from the cob and scatter over with the avocado, salad onions and dressing. Squeeze over the remaining lime juice and serve at once.