Steam the broccoli until just tender but still crisp, 2 to 3 minutes. Transfer it to a strainer or colander and rinse it briefly under cold running water to stop the cooking process. Drain well.

Meanwhile, toast the sesame seeds. Heat a small skillet over medium-low heat. Add the seeds and toast them, shaking the pan gently or stirring constantly, until the seeds are lightly browned.

In a medium bowl, gradually whisk together the mustard with the vinegar and oregano and then the oil. Add the broccoli, tomatoes and onion, and toss to coat them. Season to taste with salt and pepper. Transfer to a serving plate and sprinkle the toasted sesame seeds over the salad.