Preheat the oven to 200 degrees. Mix the sweet potato cubes with half a tablespoon of olive oil and a pinch of cinnamon. Distribute this evenly over a baking sheet.
Roast the sweet potato in the oven for 20 to 25 minutes until tender. Towards the end of the cooking time, heat the remaining olive oil in a frying pan and briefly toast the fennel seeds. Place the mackerel fillet skin-side down in the pan and cook until the fish is almost done. Quickly flip the fillet over and remove the pan from the heat.
Remove the mackerel from the pan and set aside. Add the spinach directly to the warm cooking fat in the same pan and let the leaves wilt briefly.
Distribute the roasted sweet potato and warm spinach over a plate. Place the pan-seared mackerel fillet on top and serve immediately.