Cut red onion in half (so you get half a ring), slice as thin as possible, and place in medium sized bowl. Sprinkle with a dash of salt and toss to combine.
Pour the olive oil over the tomatoes and onion and toss well. Add a few grinds of fresh black pepper and let sit at room temperature for 20-30 minutes while the flavors meld and the juices from the tomatoes begin to extract to make a fabulous dressing.
Before serving, add the balsamic vinegar, liberally crumble the blue cheese and toss everything well to combine.
Taste the salad and if it needs a bit more salt and pepper or you think it needs more balsamic vinegar, add it now. Plate and sprinkle a few crumbles of blue cheese on top.