For the Caramelized Onions: in a large saucepan over medium-low heat, heat the olive oil. Add the onion, rosemary, and sugar and cook, stirring occasionally, until onions are deeply browned and caramelized. Add the sherry vinegar and cook another 2 minutes. Season with salt and pepper to taste.
For the Manchego Queso: In a medium sized pot, heat sweet white rice flour and butter, stirring occasionally, over medium heat until butter is melted and butter/flour mixture starts to bubble, about 3 to 5 minutes. Add milk, and stirring constantly, heat until mixture thickens, about 8 minutes. Fold in cheese and stir until melted. Add salt and white pepper.
To serve, layer half of the caramelized onions in a serving dish and pour Manchego Queso over the onions. Top with remaining onions.
Recipe by Allons-Eat! at https://www.allonseat.com/caramelized-onion-and-manchego-queso/