Drain the corn and beans. Peel and finely dice the onions. Clean and wash the spring onions and cut into fine rings. Clean, wash and dice the pepper.
Put everything in a bowl. Drain and add the tuna. Finely chop the parsley and add it as well. Mix all ingredients well.
For the dressing mix the vinegar with the mustard, season with salt, pepper and sugar and stir in the oil. Since the tuna is salty, add salt sparingly. Pour the dressing over the salad and let it steep overnight.
This salad tastes very good with baguette, for grilling or as a filling for wraps. Depending on your taste, you can vary the vegetables and e.g., omit the red onions or the peppers.