Step 1: Begin by preparing the pickled red onion. Using a mandoline or a very sharp knife, thinly slice the red onion into uniform, paper-thin rings. In a small bowl, combine the sliced onion, white wine vinegar, and sugar. Mix well and set aside to quick pickle for at least 5 minutes, stirring occasionally.

Step 2: While the onion pickles, prepare the mackerel pâté. Carefully remove any skin or bones from the smoked mackerel fillets. In a high-speed blender or food processor, combine the mackerel, light cream cheese, lemon juice, and half of the fresh dill. Blend until completely smooth and creamy, scraping down the sides as needed. Season with black pepper to taste. For a true 'HARD' difficulty, achieve a perfectly emulsified, velvety texture.

Step 3: Slice the artisan bread into 1 cm thick pieces. Lightly brush both sides of each slice with olive oil. Heat a grill pan or a cast-iron skillet over high heat until smoking. Carefully grill the bread slices for 1-2 minutes per side, pressing down lightly, until golden brown with distinct char marks and a crispy exterior. This requires precise timing to avoid burning.

Step 4: To assemble, spread a generous layer of the smoked mackerel pâté onto each grilled bread slice. Drain the pickled red onion thoroughly and artfully arrange a small tangle on top of the pâté. Finely chop the remaining fresh dill and precisely sprinkle over each crostini for garnish. Serve immediately.