Cut the bacon into 1-inch dice. Put into an iron skillet and cook on low heat until all the bacon and has been rendered, stirring occasionally. Drain on paper towels. Reserve 3 tablespoons for the dressing, save the rest in the refrigerator. Do not remove the fat from the pan.

In a large bowl put the spinach, grapefruit, mushrooms, pecans, bacon, and blue cheese.

With the heat on very low, add the mustard to the pan and whisk briskly. Still whisking, add the vinegar, turn off the heat and add the sugar, whisking all while. Taste for seasoning - if it is too sour, add more vinegar, to sweet, add more vinegar.

Pour the mixture over the spinach and toss to combine. Put into a nice bowl and serve.