To prepare a tuna tarter on cucumber by roku recipe, you'll need: Tuna, cucumber, sour cream, mayonnaise, lemon juice, salt and pepper to taste. Preheat the oven to 375?F (190?C) and grease two tart tins with cooking spray. Peel the cucumber and slice it in half lengthwise. Place the slices on the rimmed baking sheets. Drizzle with olive oil and sprinkle with salt. Bake the tarts for 20 to 25 minutes or until the tops are golden brown. Serve warm.