Preheat your grill to its highest setting. Line a grill pan or tray with foil. Halve the cherry tomatoes and tumble them onto the tray. Slide under the grill for 5-8 mins till popped and slightly charred. Set aside. Turn the grill off.
While the tomatoes grill, peel and thinly slice the onion. Peel and crush the garlic. Peel and thinly slice the sweet potato into ½cm half-rounds. Thinly slice the rainbow chard stems and roughly chop the leaves.
Pop the sun dried tomatoes in a heatproof bowl and cover with boiling water. Set aside to soak.
Place a large frying pan (ovenproof if possible) on a medium heat for a few mins. When warm, add 1 tbsp olive oil and the onions. Fry for 5 mins till just softened. Add the garlic and sweet potato slices. Gently fold together. Pop on a lid and leave to cook for 10 mins, till the potato has softened.
Beat the eggs and mix in the crème fraîche. Zest in the lemon and season generously. Add the thyme leaves. Drain and squeeze the sun dried tomatoes. Thinly slice them. Heat the grill to medium-high.
When the potatoes are tender, add the chard stems to the pan. Place the lid back on for 2 mins. Then add the chard leaves, grilled tomatoes and sun dried tomatoes. Fold everything together gently. Pour over the egg mixture, shaking the pan a little so that everything is covered.
Cook on a medium heat for 5 mins till the edges are beginning to set. Slide under the grill for 8-10 mins till the frittata is golden and set.
While the frittata is cooking, juice half the lemon into a medium bowl. Whisk with ½ tbsp olive oil and some seasoning. Trim the courgette and using a veg peeler, create ribbons, turning the courgette as you go. Stop when you get to the centre and finely chop it. Toss through the lemony dressing.
Remove the frittata from under the grill and leave to rest for a couple of mins. Serve in slices with the lamb's lettuce topped with the lemony courgettes.