Method
1
Preheat and prepare the vegetables
Heat the oven to 200°C conventional or 180°C fan. Use a wide roasting tin, roughly 35 × 25 cm or larger, so the food fits in a single layer; use two tins if crowded. Scrub the sweet potatoes, leaving the skin on if liked, and cut into 2.5 cm chunks. Cut the onions into broad wedges and lightly crush the peeled garlic. Wash the lemon, zest half, measure 15 ml juice and cut the remainder into wedges.
2
Crush the seeds and season the chicken
Lightly crack 2 g fennel seeds with a pestle and mortar, leaving some seed shape; no soaking or fine grinding is needed. Do not wash the raw chicken. Pat dry with kitchen paper, then coat both sides with 15 ml oil, half the seeds, 2 g salt, half the pepper, the measured lemon juice and zest. Do not leave it marinating at room temperature for a long period.

3
Arrange the tin
Toss the sweet potatoes, onions and garlic in the tin with the remaining 15 ml oil, seeds, 2 g salt and pepper. Arrange in one layer and place the legs between or partly over the vegetables, skin-side up with gaps between them. Add lemon wedges in the spaces. Clean hands, utensils and surfaces that contacted raw chicken, and do not reuse its plate for cooked food.
4
Roast until cooked through
Roast on the middle shelf for about 45–55 minutes. Turn the vegetables after 25–30 minutes, keeping the chicken skin-side up. If the onions or skin darken too early, loosely cover with foil and continue cooking. A fork should slide easily into the sweet potato. Check the thickest chicken with a clean probe, avoiding bone: its centre must reach 75°C for 30 seconds. If it has not, continue and check every 5–10 minutes. Golden skin alone does not show that the centre is cooked.

5
Rest and serve
Remove the tin using clean tools and rest for about 5 minutes. Serve one leg and a quarter of the vegetables per person with a little pan juice. Squeeze a little roasted lemon over it if liked; discard the tough peel and pips rather than eating the whole wedge.
Cooking tips
- Fennel seeds have a sweet, anise-like aroma and differ from cumin and star anise. Use 2 g, not the entire 50 g pack.
- Legs substantially larger than about 250 g each will usually need longer. Use the temperature check. If cooked vegetables are removed first, use clean tongs and a clean plate.
- Cut the sweet potato evenly; a crowded tin affects browning and heating. Avoid changing the chicken cut, size and temperature all at once on a first attempt.
Allergens
The Xianyitang pack states that it is packed in a facility that also handles peanuts, tree nuts, sesame and soya. Check the actual pack for anyone with allergies.
Storage and reheating
Divide leftover chicken and vegetables into shallow containers, cool promptly and refrigerate within 2 hours of leaving the oven. Keep at 5°C or below and eat within 48 hours or freeze. Reheat only once until both the centre of the chicken and the vegetables are steaming hot.