For the salad:
1) Heat a medium frying pan over high heat, add the chorizo, and cook until golden brown. Remove with a slotted spoon to a plate lined with paper towels.
2) Place the greens and tomatoes in a large bowl and toss with half of the vinaigrette. Season with salt and pepper, to taste, and divide among 4 large plates.
3) Spread the blue cheese on the croutons. Scatter the chorizo over the salads and garnish each plate with remaining vinaigrette and 2 croutons; serve.
For the vinaigrette:
1) Whisk together vinegar, mustard, and salt and pepper in a small bowl. Slowly whisk in the oil until emulsified.
For the croutons:
1) Brush the bread with the oil and arrange on a baking tray. Sprinkle with salt and pepper and bake until lightly browned on both sides, about 7 minutes. Let cool.