To make the garlic aioli, add mayonnaise, lemon juice, garlic, salt, and pepper to a medium bowl and stir to combine. Let chill for about an hour before serving. Transfer to a serving bowl. Garnish with fresh dill.
Open tinned fish and place cans on a tray or cutting board. Place cornichons, mustard, pepperoncinis, olives, and radishes in small bowls and place next to tinned fish. Add bread and crackers. Serve with aioli.