Combine sugar and cinnamon.
Melt butter in a nonstick skillet over medium-low heat, tilting pan to coat the bottom.
Place one tortilla into pan. Layer with the 1/4 cup gouda, apple slices, walnuts and cinnamon-sugar mixture.
Top with remaining gouda and tortilla. Cook for 2-3 minutes on each side or until cheese is melted.
Transfer quesadilla to a cutting board. Cut into wedges.