Set a large pan over a medium heat. Rinse the chard and finely shred it. Add it to the pan with a splash of water and a good pinch of salt. Cook the chard for 5-8 mins till it's wilted.
Spoon the chard into a colander or sieve. Set in the sink and run under cold water till it's cool enough to handle. Gently squeeze out any excess liquid. Roughly chop the chard.
Whisk your eggs in a large bowl with a good pinch of salt and pepper. Finely grate in the zest from the lemon. Add the chopped chard. Halve, peel and thinly slice the red onion. Add to the bowl. Crumble in half your pack of feta (see our tip below for what to do with the rest). Roughly chop the dill, discarding any thick stalks, and add it. Fold everything together.
Heat your grill to high. No grill? Just heat your oven to its highest setting. Set a small ovenproof frying pan over a high heat. Brush the pan with 2 tsp oil. Spoon the egg mix in. Lower heat. Cook for 2 mins or till you can see the fritatta setting on the bottom and around the sides.
Pop under the grill or on the top shelf of the oven. Cook for 5-10 mins or till puffed up, set and golden on the top.
While the frittata grills, halve the tomatoes. Pop in a bowl with the salad leaves. In a separate bowl, squeeze in the juice from half the lemon. Add 1 tsp Dijon mustard and 2 tsp olive oil. Whisk with a pinch of salt and pepper then tip into the salad bowl and toss to coat the leaves and tomatoes in the dressing. Serve the salad alongside wedges of the fritatta.