Preheat the oven to 400°F (200°C).
Crack the eggs into a medium-large mixing bowl and whisk until well beaten and slightly frothy, about 2 minutes.
Whisk in the heavy cream, whole milk, chopped scallions, chopped dill, black pepper, and salt until combined; set the egg mixture aside.
Melt the butter in a large ovenproof skillet over medium heat.
Pour the egg mixture into the skillet, then quickly and evenly arrange the thinly sliced red onion, cubed cream cheese, smoked salmon strips, tomato slices, and capers on top of the eggs.
Cook on the stovetop over medium heat without stirring for 4 minutes to set the bottom.
Carefully transfer the skillet to the preheated oven and bake until the top is golden and the frittata is set, about 35 minutes.
Remove from the oven and let the frittata cool for 5 minutes, then slice and serve warm.