Slice brie into thin slices. I don't include the rind in the sandwiches.
Make the honey mustard sauce by whisking together spicy brown mustard, honey and hot sauce in a small bowl. Adjust honey and hot sauce to taste.
Spread a thin layer of mustard sauce on a slice of sourdough (feel free to spread it on both pieces of bread). Top with brie slices, apple slices, a handful of spinach and the remaining sourdough slice.
Heat a large skillet over medium heat. Add the butter. Once melted, add one grilled cheese sandwich. Cook on one side for 3-4 minutes, until golden and crisp. Flip and cook another 3-4 minutes. Decrease heat to medium-low if the sandwich is browning too quickly. Enjoy sandwiches immediately.