Combine all ingredients in a medium bowl and season with salt and pepper to taste.
Black Angus Steak:
Preheat a grill pan over high heat until smoking. Brush the steaks with olive oil and season with salt and pepper, to taste. Grill for 4 to 5 minutes on one side until golden brown, turn over, reduce heat to medium and continue cooking for 5 to 6 minutes for medium-rare doneness.
Tomato-Red Onion Salad:
Whisk together the vinegar and mustard in a small bowl. Slowly whisk in the olive oil and season with salt and pepper to taste. Toss the watercress in a few tablespoons of the dressing and arrange on a platter. Top with slices of the tomatoes and onions and drizzle with the remaining dressing. Sprinkle with the blue cheese.