In a medium skillet, melt the butter over medium heat. Be careful not to let it burn.
Add grapes and stir. Add walnuts and sugar. Let simmer for about 10 minutes, or until syrup starts to form. Using a slotted spoon, remove the grapes and walnuts from the skillet and reserve in a bowl. Return the skillet to a medium-low heat and reduce remaining liquid to a thick syrup consistency.
In a separate pan, heat oil and fry the tortillas until just crispy. Remove from oil with a slotted spoon or tongs and let rest on a flat plate lined with paper towels to absorb the oil.
To assemble, apply a generous amount of brie cheese to the top of a tostada or fried tortilla. Add 2 spoonfuls of roasted grapes & walnuts.