Prepare the Onion: Heat the olive oil in a large, oven-safe skillet over medium heat. Add the chopped onion and cook until softened and translucent, about 5 minutes.
Whisk the Eggs: In a large bowl, whisk together the eggs, crème fraîche, dill, salt, and pepper until well combined.
Add the Smoked Trout: Stir the flaked smoked trout into the egg mixture, being careful not to overmix.
Cook the Frittata: Pour the egg mixture into the skillet with the onions and cook over medium heat for 5-7 minutes, until the edges are set but the center is still slightly jiggly.
Finish in the Oven: Transfer the skillet to a preheated oven (350°F) and bake for 10-12 minutes, or until the frittata is fully set and lightly golden on top.
Serve and Garnish: Let the frittata cool slightly before slicing. Serve with a dollop of crème fraîche and a sprinkle of fresh dill on each slice.