Start by gathering all your ingredients. Lay them out on your kitchen counter. You’ll need your sourdough bread, Gruyère cheese, blueberries, sugar, and mayo. This ensures you won’t run around the kitchen halfway through your sandwich preparation.

In a small bowl, mix the mayonnaise and Dijon mustard. This creamy concoction will add flavor and moisture to your sandwich. Honestly, don’t skip this step; it brings everything together beautifully.

On a slice of sourdough, spread a generous layer of the mayo-mustard mix. Then, layer in the cheese followed by a handful of fresh blueberries. Sprinkle the sugar on top of the blueberries. The sugar will help to balance out the flavors and enhance the sweetness of the blueberries as they cook.

Next, take a few leaves of fresh basil and tuck them in the sandwich. This adds an aromatic element that's hard to resist. Basil and blueberries are an underappreciated combo that works wonders together!

Place another slice of sourdough on top, creating a sandwich. Heat a skillet over medium heat and lightly butter the outside of your sandwich. Once the pan is hot, place the sandwich in the skillet. Grill for about 3 to 4 minutes per side, or until golden brown and crispy.

Once both sides of the sandwich are perfectly golden and the cheese has melted, remove it from the pan. Slice the sandwich in half, and there you have it—a blueberry grilled cheese that looks as good as it tastes!