01 - Preheat oven to 350°F (175°C); grease a 9x13-inch baking dish thoroughly.
02 - Distribute the bagel cubes evenly in the greased baking dish, ensuring complete coverage.
03 - Scatter cream cheese cubes, smoked salmon, red onion, cherry tomatoes, chives, and capers evenly over the bread base.
04 - In a large mixing bowl, whisk together eggs, whole milk, kosher salt, black pepper, and dill if using, until fully combined.
05 - Pour the egg mixture evenly over the layered ingredients. Press down gently on the bagel cubes to help them soak up the liquid.
06 - Evenly sprinkle cheddar and mozzarella cheese across the surface.
07 - Allow the casserole to rest for at least 15 minutes at room temperature or refrigerate overnight for optimal texture.
08 - Transfer to the oven and bake for 40–45 minutes until set in the center and golden brown on top.
09 - Let cool briefly before slicing; garnish with additional chives as desired.