For the croutons: Preheat the oven to 350 degrees F. Put the bagel pieces in a large bowl, drizzle with the olive oil, and toss to coat. Add the poppy seeds, sesame seeds and some salt; toss again. Spread the bagel pieces out in a single layer on a baking sheet and bake until light golden brown and crispy, 8 to 10 minutes.
For the vinaigrette: Put the lemon zest and juice, dill, capers, mustard, and some salt and pepper in a medium bowl; whisk to combine. Slowly whisk in the olive oil to make a thick creamy dressing. Taste, and season with salt and pepper.
For the salad: Put the croutons, smoked salmon and cream cheese in a large bowl, and then mound the spinach, fennel, cucumber and red onion on top. Spoon some of the vinaigrette around the side of the bowl and toss gently to coat the salad with dressing. Spoon the remaining vinaigrette nicely on individual serving plates and gently mound some salad on top. Serve immediately.