Spinach Salad with Smoked Trout and Tart Apple

1/2 tart apple such as Granny Smith 1 1/2 tablespoons extra-virgin olive oil 1 tablespoon fresh lemon juice 1/4 teaspoon Dijon mustard 4 cups packed spinach (about 6 ounces), washed well, dried, and coarse ;stems discarded 1 smoked trout* (about 8 ounces), skin and bones discarded and fish ;broken into bite-size pieces 3 tablespoons sliced almonds, toasted golden 3 tablespoons dried currants *available at deli counter of many supermarkets