For the "everything" seasoning: In a small bowl, stir together the sesame seeds, poppy seeds, dried garlic, dried onion and salt. Set aside.
For the waffles: Coarsely grate the potatoes on a box grater. Wrap the grated potatoes in a clean kitchen towel and squeeze out the excess moisture. Whisk the flour, baking powder, salt and 1 tablespoon of the "everything" seasoning together in a large bowl. Add the potatoes, eggs, milk, butter and scallions; stir to combine.
Preheat a waffle iron. Using 1/3 cup batter per waffle, cook the waffles for about 4 minutes (or according to manufacturer's directions). With an offset spatula, flip the waffles and cook an additional 2 minutes, or until they're a deep golden brown. As you go, transfer the cooked waffles to a baking sheet and keep warm in a low oven.
To serve: Top the waffles with smoked salmon and a dollop of creme fraiche, sprinkle with capers and "everything" seasoning, and garnish with a dill sprig.