Step 1Heat the oven to 450 degrees. Heat the grill to medium-high.
Step 2Cut the tomatoes in half and place in a baking pan, cut side up. Drizzle with 2 tablespoons of the olive oil, scatter with the garlic and season with salt and pepper. Roast, stirring once or twice, until the tomatoes have softened and smell great, about 20 minutes.
Step 3While the tomatoes roast, season the steak with salt and pepper and grill until medium rare about, 3 to 4 minutes each side. Set aside.
Step 4Toast the bread while the steak cooks. Set aside.
Step 5Combine the mustard, horseradish and vinegar in a small bowl. Whisk in the remaining 3 tablespoons of olive oil and season with salt and pepper. Toss the vinaigrette with the watercress.
Step 6To assemble the sandwiches, slice the steak as thinly as possible and divide among 4 of the bread slices. Top the steak with the roasted tomatoes and the watercress, then the remaining bread slices.