Place the lentils and water in a pot, cover and bring to a boil. Reduce to a simmer and cook for around 20 minutes until the lentils are just tender and have absorbed most of the water. Drain off any leftover water and set aside to cool.
As the lentils are cooking, prepare the rest of the salad. Slice the cucumber, halve the tomatoes (or quarter, if larger) and cut the avocado into small bite-sized pieces.
Bring a medium-sized pot of water to a simmer and cook the asparagus for around 2 minutes to gently cook, then remove the asparagus and run under cold water or place in a bowl of cold water to preserve color and stop cooking. Don't discard the water.
Trim the skin and any fatty or dark areas from the salmon. Cut to even thickness pieces, as far as possible.
Squeeze the ½tbsp lemon into the water you used for the asparagus, bring back to a simmer, if not still hot, then add the salmon pieces. Keep the water just simmering and poach for around 6-8 minutes until the salmon is just cooked through and will flake. Remove the salmon and set aside.
Drizzle roughly 1 tablespoon of oil and ½tbsp lemon over the drained lentils, add a little salt and pepper and stir to mix. Divide the lentils between serving plates then top with chunks of salmon, cucumber, tomato, and avocado. Place some asparagus to one side of the place. Drizzle over the remaining olive oil, lemon and a little salt and black pepper then serve.