In a pot or large skillet, heat cooking oil over medium-high heat. Saute onion for 3-5 minutes or until softened. Stir regularly.
Stir in the leek and garlic. Cook until leeks starts to soften, about 3-5 minutes. Add a little bit of ghee/butter or cooking oil if needed. Stir regularly.
Meanwhile, bring a pot with potatoes and lightly salted water to a boil. Cook potatoes until tender.
Preheat oven to 350F or 180C.
When leeks are softened, add celery and carrots and cook for 3 minutes more. Stir regularly
Increase heat to high and add white wine or vegetable stock. Stir well and reduce heat. Simmer for 3-5 minutes. Season with dill, sea salt, and black pepper to taste.
Transfer veggie mix, juice included, to an oven-safe dish. Push veggies to the side and place fish in the middle. Season fish with black pepper, a little salt and top with a few lemon/lime slices
Cook in the oven for 15 minutes or until fish is cooked through.
Mash potatoes, season with ground nutmeg, salt, and black pepper to taste. Optional: add a little water, an egg, or milk to improve consistency.