Prep the ingredients and add them to a bowl. Gently mash/stir together. Give it a taste and adjust ingredients as needed (e.g. add more lemon juice or salt & pepper).
Serve in a sandwich, wrap, lettuce wraps, etc.
Notes
Makes about 2 sandwiches (up to 4 if you don't fill them much). To halve the recipe, use 1 can of mackerel and halve each ingredient (it's fine to eyeball this recipe... it's very flexible).
Mackerel taste/quality can vary a lot from brand-to-brand. The best one I've tried is King Oscar.
Skip the cucumber if you don't plan on eating it right away (it'll make it more watery if it sits).