Cook the bacon in a medium skillet over medium heat, stirring occasionally, until crisp, 9 to 11 minutes. Remove to paper towels; reserve the drippings in the skillet.
Whisk the vinegar, sugar, mustard and 1/4 teaspoon each salt and pepper into the drippings. Slowly whisk in the olive oil until emulsified.
Place the spinach in a large bowl and pour the warm dressing on top; add the grapes, blue cheese and the reserved bacon and toss. Season with salt and pepper. Top with croutons.
Photograph by Johnny Miller
Courtesy of Food Network Magazine