1 For the vegetables: peel and rinse potatoes, cut into 1 cm (approximately 1/2 inch) cubes. Cook in salted boiling water for about 10 minutes. Remove with a slotted spoon and drain on paper towels. Heat 2 tablespoons of butter in a pan and cook potatoes until golden brown.
2 For the fish: rinse mackerel, pat dry and sprinkle with lemon juice inside and out. Spread garlic butter in the abdominal cavities and season with salt and pepper. Score skin 2-3 time and brush with olive oil. Arrange mackerel in a fish grill grid and grill for about 10-12 minutes on the hot grill, turning often.
3 Peel cucumbers and halve lengthwise, scrape out seeds with a spoon. Cut pulp into pieces. Peel shallot and chop finely. Heat remaining butter in a pan and saute shallot until translucent. Add honey and caramelize briefly, add mustard seeds and deglaze pan with wine and broth. Season with salt and pepper and simmer for about 10 minutes on low heat. Rinse dill, shake dry and chop finely. Add sour cream, mustard and dill to cucumbers and season with lemon juice, salt and pepper.
4 Drain potato cubes on paper towels and add to braised cucumbers. Arrange vegetables in small bowls. Place grilled mackarel on plates and serve with vegetables. Garnish with lemon wedges and dill, serve.