Preheat the grill to its highest setting.
Place mackerel fillets flesh-side up on a grill tray and season with cardamom, horseradish, salt, and pepper.
Grill the fish for 4 minutes, then flip and grill for another 30 seconds.
Boil potatoes in a pan until tender, then drain, cool, and peel.
Boil cauliflower in salted water for 3 minutes.
Add green beans to the same pan and cook with cauliflower for 3 more minutes before draining.
Sauté olive oil, anchovies, capers, and garlic in a hot frying pan for 1 minute.
Mix the cooked potatoes, cauliflower, and green beans into the frying pan.
Serve the salad topped with mackerel and drizzled with pan juices.