- In a large pot of salted boiling water, cook the celeriac for 18 minutes. Add the garlic and green onions. Continue cooking for 2 minutes. Remove from the heat. Stir in the parsley. Drain.
- In a food processor, purée the celeriac mixture with the butter until smooth. Season with salt and pepper. Keep warm.
- Off the heat, add the oil to a large non-stick skillet. Place the mackerel pieces skin side down in the skillet. Season with salt and pepper. Place a plate or another skillet on top of the fish to keep it flat while cooking. Cook over medium-high heat for 4 minutes or until the skin is nicely golden. Remove the plate. Turn the fillets over and continue cooking for 1 minute, without weighing them down. Set aside on a plate.
- In the same skillet over medium-high heat, brown the green onions for 1 minute. Season with salt and pepper.
- Divide the celeriac purée among four plates. Top with the fish, green onions, kohlrabi, watercress and a drizzle of olive oil. Season with pepper.
Eco-friendly tip: Because mackerel reproduce quickly and have a low impact on ecosystems, Canada's Atlantic mackerel fishery has consistently been rated a "best choice" for consumers.