Preheat the oven to 475 degrees F. Line a large cookie sheet with foil.
In a bowl, toss the cucumbers with the vinegar, 1 tablespoon dill, 1 tablespoon oil, 1/8 teaspoon salt and 1/8 teaspoon pepper.
In a small bowl, combine the panko, horseradish and the remaining dill and oil. Sprinkle the salmon with 1/8 teaspoon salt and 1/8 teaspoon pepper, and place on a cookie sheet, smooth-side up. Press the panko mixture evenly on top of the fillets. Bake until golden brown on top and opaque throughout, about 8 minutes.
Toss the spinach with the cucumber mixture in a bowl and serve with the salmon.
Nutritional information per serving: About 345 calories, 36 g protein, 8 g carbohydrate, 18 g total fat (3 g saturated), 5 g fiber, 93 mg cholesterol, 300 mg sodium