Place yogurt into a fine-mesh sieve lined with a coffee filter and allow to drain, about 20 minutes. You should end up with about a cup of thick yogurt. Heat a small skillet over medium heat, add walnuts, and toast while stirring until nuts start to turn golden brown and become fragrant, 3 to 5 minutes. Allow to cool. Mix yogurt, sultanas, walnuts, dill, mint, salt, and pepper in a large bowl, add cucumber and toss to combine. Refrigerate for 1 hour.