Oily fish (salmon, mackerel, sardines, herring, fresh tuna) provides EPA and DHA omega-3 fatty acids that other foods don't match. Two servings (140g each) weekly is the NHS recommendation, supported by cardiovascular and brain health evidence.
Practical implementation
Tinned sardines or mackerel as lunch (cheap, sustainable, high in omega-3). Salmon dinner once weekly. Smoked salmon at breakfast occasionally. Fresh mackerel from fishmongers (under-rated and excellent).
If fish doesn't suit you
Omega-3 supplement (1-2g EPA+DHA daily). Algae-based for vegetarians. Less impactful than whole fish but still useful.