Transfer the cooked lentils and all of the chopped veggies into a large mixing bowl.
Drain the mackerel and flake the fillets over the salad with a fork.
Add the lemon zest to the mixing bowl, then squeeze fresh lemon juice over the ingredients.
Add a tiny drizzle of extra virgin olive oil, sprinkle a bit of sea salt and a few grinds of cracked black pepper, to taste, and toss to evenly coat everything.
Keeps in the refrigerator for three days!