Heat your oven to 200°C/Fan 180°C/Gas 6. Place the tomatoes on a baking tray. Drizzle with 2 tsp olive oil and season. Bake in the oven for 20 mins. After 20 mins, turn the oven down to its lowest setting to keep the tomatoes warm.
Meanwhle, halve and peel the onions. Finely chop. Heat a frying pan for 1 min. Drizzle in 1 tbsp olive oil and add the onion. Sprinkle in a good pinch of salt. Cook on a low heat for 10 mins till soft and translucent. Stir now and then. Transfer to a large bowl and leave to cool slightly. Wipe the pan clean.
While the onion cooks, crumble half the pack of feta into a separate bowl. Spoon in 2 tbsp Greek yogurt. Zest in the lemon. Squeeze in the juice from half the lemon. Crack in plenty of pepper. Mash with a fork till combined and smooth.
Finely shred the mint leaves. Add most of it to the bowl with the onions. Tip in the flour and 1 tsp baking powder. Season well. Using a fork, stir everything together to coat in the flour.
Make a well in the centre of the onions. Beat the egg in a bowl, then pour it in. Gently stir everything using a fork. Trickle in 200ml sparkling water (or still water if you prefer), stirring constantly. The bubbles will make the pancakes extra fluffy. The batter should be thick but easily drop off the fork.
Place the pan you cooked the onions in back on a medium heat. Drizzle in 2 tsp olive oil and swirl it around the pan. Drop in 2 tbsp of batter to make a pancake the size of an American pancake. You should be able to cook 2 or 3 at the same time.
Cook the pancakes for 3 mins. Using a spatula, flip it over and cook for a further 3 mins. They should be golden on the outside and fluffy in the middle. Place the cooked pancakes on a baking tray. Slide into the oven under the tomatoes to keep warm while you cook the rest.
Whisk 2 tsp oil with the remaining lemon juice and toss through the lettuce. Stack the pancakes onto 2 plates and top with a dollop of the whipped feta. Serve with the tomatoes, salad and the remaining yogurt. Scatter with reserved mint.