On baking sheet, toast walnuts on 350°F until light golden, 6 to 8 minutes. Let cool.
In a small bowl, combine yogurt, 4 Tbsp cheese, and lemon juice, mashing with fork until almost smooth.
Stir in chives, hot pepper sauce, salt and pepper; set dressing aside.
Mix apple, kohlrabi and arugula; toss with dressing to coat. Garnish with walnuts and remaining cheese.
