Whisk the eggs in a bowl and season generously with salt and pepper. Set aside. Sauté the onions and garlic in olive oil until just softened. Set aside.
Heat 1½ tsp butter in a 20cm non-stick pan over a moderate heat, then add ¼ of the eggs mixing, swirling to coat. Move the mixture gently with a spatula, then allow it to almost set. Put some trout, spinach and ricotta on one side, then fold the other side over. Invert onto a plate, then top with fennel, dill and crème fraiche. Repeat with the remaining ingredients to make four omelettes.