Place the onion slices in a small bowl. Pour the vinegar over the top and season to taste with salt and pepper. Allow this to sit for about ten minutes. The process takes the bitterness out of the onion.
Lay out the baguette sections on a work surface. Place one ounce of cheese on one side of each baguette. Spread one tablespoon of English pickle atop the cheese slices. Sprinkle the sundried tomatoes over the pickle.
On the other side of each baguette, layer on baby spinach leaves. Top the spinach with onion slices.
Close the sandwich and serve.
Note: If packing for take-away, wrap each sandwich carefully and consume within a few hours.