To make the sauce: in a small bowl, combine the sour cream, lemon juice, mustard, and milk. Set aside.
Lightly grease a deep, large nonstick sauté pan with vegetable oil; fill pan halfway with water. Bring to a boil, then reduce heat to a simmer. Break one egg into a measuring cup; carefully slide egg into the water, holding lip of the cup as close to the water as possible. (Repeat with remaining eggs, allowing a decent amount of space in between each one.) Simmer, uncovered, for 3-4 minutes or until whites are set and yolks begin to thicken but are not hard. Use a slotted spoon to remove eggs.
Top muffin halves with smoked salmon and a poached egg. Spoon sauce over the top and sprinkle with red onion, scallions, and black pepper to taste. Serve immediately.