Preheat the oven to 400° F.
Place eggs, heavy cream, whole milk, and the Dijon into the barrel of a food processor. Pulse 8 to 10 times or until eggs are frothy and ingredients are well incorporated. Add salt and pepper and pulse 2 more times.
Add butter to a heavy cast iron or other oven-proof skillet over medium heat. Add the leeks and the red onion and sauté until soft, about 3 minutes. Add the shallot and sauté 1 minutes more. Remove the skillet from the heat.
Using a metal spatula, loosen the leeks and onions and shallot is there is any sticking.
Scatter the chives, half the dill, any add-ins (I am using leftover hash brown potatoes which I have brought to room temperature), the smoked salmon, the cream cheese cubes, and half the capers.
Pour the egg mixture over the salmon and other ingredients in the skillet. Top with the crumbled goat cheese.
Cook for 3 minutes over medium heat, undisturbed. Avoid the urge to move or stir the eggs in any way. Carefully transfer the skillet to the preheated oven.
Bake until the top is golden brown and the frittata is set; about 35 minutes. Cool for 5 minutes, then garnish with the remaining torn dill. Cut and serve warm.