Heat the olive oil in a wide, heavy-based pan over a low-medium heat. Add the onion, celery and a pinch of salt. Cook gently for 8-10 minutes until soft. Do not colour.
Add the garlic, parsley and chilli flakes. Cook for 2 minutes, stirring.
Add the chard. Stir to combine, cover and cook over a low heat for 8-10 minutes until wilted.
Raise the heat to medium-high. Add the squid and cook for 3-4 minutes, stirring, until opaque.
Pour in the white wine. Let it bubble and reduce for 2-3 minutes until the alcohol has cooked off.
Add the crushed tinned tomatoes. Stir well, season with salt and pepper, reduce the heat to low, cover and cook for 20 minutes.
Remove the lid. If the sauce is too wet, raise the heat and cook uncovered for a further 3-4 minutes to reduce.
Scatter over the marjoram leaves. Taste, adjust seasoning and serve immediately with toasted bread.
Notes
Calamari releases liquid as it cooks - don't be alarmed if the pan looks wet after step 6. The uncovered reduction in step 7 handles this.
Marjoram is added at the end - it loses its flavour if cooked for too long.
Taggiasca extra virgin olive oil is worth using here if you can find it.