Steps
1. Preheat the oven to 400°F (200°C).
2. In a small bowl, mix olive oil, minced garlic, parsley, dill, lemon juice, and zest. Season with salt and pepper.
3. Place the mackerel fillet on a baking sheet and spread the herb mixture evenly over the top.
4. Bake in the preheated oven for about 15 minutes, or until the fish is cooked through and flakes easily with a fork.
5. Meanwhile, in a skillet, melt the butter over medium heat. Add the spinach or kale and sauté until wilted, about 3-4 minutes.
6. Serve the herb-crusted mackerel alongside the sautéed greens, garnished with lemon wedges if desired.