Cook the bacon in a medium skillet over medium heat, stirring occasionally, until crisp. 9-11 minutes. Remove to paper towels; reserve the drippings in the skillet.

Whisk the vinegar, sugar, mustard and 1/4 tsp 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.