Fill a shallow pot half way with water and bring to a gentle simmer. Add vinegar and salt. Using a slotted spoon, swirl the water around the pot so that it continues to move after you remove the spoon. Carefully crack each egg into the swirling water and allow to cook for approximately 4 – 5 minutes.
While eggs cook, toast sourdough slices. Top with smoked salmon.
Using the slotted spoon, carefully remove each egg from poaching water, allowing any excess water to drain back into pot. The egg whites should be opaque and cooked through, the yolks should still be runny.
Place eggs on top of sourdough with smoked salmon. Warm hollandaise gently and dollop on each benedict.
Top with capers and red onion and season with salt and pepper to taste. Garnish with lemon slices and enjoy hot.