Preheat oven to 275 degrees F. Using a mandoline, slice kohlrabi 1/16 inch thick. In a large bowl whisk together olive oil, cumin, coriander, and 1/4 teaspoon kosher salt. Add kohlrabi slices; toss to coat. Arrange slices in a single layer on two extra-large baking sheets. Place baking sheets on two separate oven racks near the center of the oven.
Bake 40 to 75 minutes or until chips are golden brown and crisp, rotating the pan, switching rack placement, and turning chips once after 30 minutes. Chips will begin browning quickly after 30 minutes. Start checking every 5 minutes after this, removing chips from baking sheets as they are done. Cool on a wire rack.
For dip, in a small bowl stir together sour cream, mayonnaise, 1/4 tsp. kosher salt, and 1/8 tsp. ground black pepper. Sprinkle chips and dip with rosemary and onions before serving.