For the fried mackerel with avocado and grapefruit, first cut off the peel including the white layer from the grapefruit with a serrated knife. Then run the knife between the flesh and the membrane to extract the segments. Put them in a large bowl.
Now prepare the dressing. To do this, squeeze the juice out of the remaining partitions and pour into a small jug (you will only need 2 tablespoons of juice). Add olive oil, lemon juice or vinegar, mustard and honey, whisk and season with salt and pepper. Add the red onion and the chili pieces.
Slice the fennel bulb as thinly as possible and add to the grapefruit fillets along with the avocado slices and watercress.
Heat a frying pan without fat. Rub the fish fillets thinly on both sides with peanut oil and season all over. Place skin side down in the hot pan. Fry over medium-high heat for 3 minutes until skin is crispy. Then turn over and fry for 1 more minute or until the fish is just cooked through. Remove the pan from the heat.
Pour the dressing over the salad and gently mix it in. Divide the salad among 4 plates and top each with a mackerel fillet. Serve fried mackerel with avocado and grapefruit.