In a saucepan, cook bacon over low heat to render fat. Cook until bacon is crispy. Remove bacon from pan and drain on paper towels. When cool, chop the bacon coarsely.
In the pan with the bacon drippings, add the olive oil and shallots. Cook shallots over medium heat until translucent, about 3-4 minutes. Remove from heat and whisk in vinegar, salt and pepper. Add croutons and stir to combine. Cover to keep warm.
In a large salad bowl, add spinach. Top with the red onion and blue cheese. Drizzle the dressed croutons and vinaigrette over the top and serve immediately.