Begin by preheating your oven to 350°F (175°C). This ensures that your frittata will bake evenly and thoroughly.
In a large mixing bowl, crack the 10 large eggs and whisk them together. Add in the heavy cream and whole milk, mixing until the mixture is smooth. Stir in the finely chopped scallions, dill, black pepper, and salt. Set aside.
In an oven-safe skillet, melt the 2 tablespoons of butter over medium heat. Once melted, add the thinly sliced red onion and sauté until they are soft and translucent, about 5 minutes.
Once the onions are ready, reduce the heat to low. Pour the egg mixture over the sautéed onions in the skillet. Gently stir to combine. Distribute the cream cheese cubes, smoked salmon strips, and Roma tomato slices evenly throughout the mixture.
Transfer the skillet to the preheated oven, and bake for 25-30 minutes or until the frittata is set and the top is lightly golden. Check for doneness by inserting a toothpick in the center; it should come out clean.
Once the Smoked Salmon Frittata is out of the oven, sprinkle with capers for a burst of flavor. Let it cool for a few minutes before slicing.
Slice the frittata into wedges and serve warm. It pairs wonderfully with a simple green salad or some crusty bread for a complete meal.