For the Sun-Dried Tomato Pesto

1. In the bowl of a food processor fitted with the steel blade, combine the sun-dried tomatoes with their oil, garlic, salt, red pepper flakes, sugar, basil, pine nuts, Parmigiano-Reggiano and 2 tablespoons of water. Pulse, scraping down the sides as necessary, until the mixture is finely puréed.

For the Grilled Cheese Sandwiches

1. Spread one side of each slice of bread with butter, then flip over and spread the pesto evenly over the other side of each slice. Heat a large nonstick or cast iron pan over medium heat. Place 4 slices of bread, butter-side-down, in the pan. Divide the cheese evenly over the bread, then top the sandwiches with the remaining bread. Cook until the first side is golden brown, a few minutes. Carefully flip the sandwiches and cook until the cheese is fully melted and the bread is golden brown, a few minutes more. Let cool slightly, then slice the sandwiches in half and serve.
2. Note: You'll have plenty of leftover sun-dried tomato pesto. It keeps well in the refrigerator for up to 4 days and can be frozen for up to 2 months. You can also try tossing the pesto with pasta; just keep in mind that the pesto is thick so you'll need to thin it with some of the pasta cooking water and a little olive oil when tossing it with the pasta.