Bring a small pot of water (about 2/3 full) to a boil. While water heats, mix yogurt, lemon juice, garlic, milk, 1 tsp dill, salt and pepper together until well combined. Set aside.
Toast the bread or english muffin and set aside.
Once water has reached a boil, reduce to a simmer. Add vinegar. Crack egg into a ramekin or cup, and slowly pour into the simmering water (if you're comfortable doing so, you can also crack the egg directly into the water). Let simmer for 3 minutes.
Carefully remove egg from pot with a slotted spoon and set aside on a paper-towel lined plate.
To assemble, spread a small amount of yogurt sauce on the toast/english muffin. Add cucumber slices and salmon, and top with poached egg. Drizzle yogurt sauce over top. Garnish with remaining dill, a dash of pepper and cayenne pepper (if desired).