Fry the bacon in a pan until it's crispy. Drain on paper towels. Let cool, crumble, and set aside.
Wipe out the pan with paper towels and heat the olive oil over medium heat. Cook the bread cubes, until lightly browned, for about 7 minutes. Drain on paper towels. Set aside.
Now let's make the dressing. Whisk the yogurt, vinegar, and lemon juice in a small bowl. Add the buttermilk and whisk again. Add the blue cheese and season with salt and pepper to taste.
In a large bowl, add the spinach, onion, and half of the bread cubes. Toss together. Pour half of the dressing and toss once again. Arrange the bacon, tomatoes, and the remaining bread cubes over the salad and drizzle with the remaining dressing. Enjoy!