Heat your oven grill to high. No grill? Just heat your oven to its highest setting.
Set a large ovenproof pan over a medium heat. Rinse your chard and finely shred it. Add it to the pan with a splash of water and a good pinch of sea salt. Stir the chard till it's just about wilted down.
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. Zest in 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 marjoram and add it. Fold everything together.
Set a small 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. Halve the tomatoes. Squeeze in the lemon juice. Mix with a pinch of salt and pepper. Serve alongside wedges of the fritatta.