Swedish toast Skagen (prawn toasts)

Ingredients

• 200-250g raw shelled prawns, tails on
• 3 tablespoons sour cream
• 1 tablespoon mayonnaise
• 2-3 tablespoons chopped fresh dill, plus extra to serve
• 2 tablespoons lemon juice, plus lemon wedges to serve
• 1 tablespoon finely chopped onion
• 1 teaspoon grated horseradish (optional)
• 4 slices rye or brioche bread, crusts removed, buttered on both sides
• 2 tablespoons fish or salmon roe, to serve (optional)