Slice the bagel in half and toast it to your desired crispness.
Spread an even layer of cream cheese on each toasted bagel half using a butter knife.
Layer the smoked salmon over the cream cheese on the bottom half of the bagel.
Add the tomato slices, red onion slices, and capers on top of the smoked salmon.
Top with arugula or mixed greens if desired, then place the top half of the bagel on the assembled ingredients.
Slice the bagel in half if serving as a sandwich, and enjoy immediately.
Notes
Feel free to customize this bagel with additional toppings such as avocado slices or a squeeze of lemon juice. For extra flavor, sprinkle with a pinch of dill or black pepper.