Scatter the chunks of sweet potato in a shallow, ovenproof dish with the onion. Add the oil, thyme and a little salt and mix together.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 40–45 minutes, turning once or twice, until the potatoes are just tender and beginning to brown.
Stir in the tomatoes. Fold each mackerel fillet in half, skin-side out, and place on top of the potatoes. Return to the oven for a further 12–15 minutes, or until the fish is cooked through.
Meanwhile, mix together the yogurt, herbs and a little salt and pepper to make a raita. Transfer the fish and potatoes to warm plates, spoon over the raita and serve with lemon wedges.