Preheat the oven to 250°. On 2 large rimmed baking sheets, toss the tomato slices with the oil. Arrange in a single layer, season with salt and pepper and sprinkle with the thyme leaves. Bake for about 1 hour and 30 minutes, until the tomatoes are very firm and partially dried; rotate the sheets halfway through baking. Let the tomatoes cool slightly, then coarsely chop them.
In a bowl, combine the chopped tomatoes, chopped thyme, parsley and the balsamic vinegar. Season with salt and pepper.
In a small skillet, toast the mustard and coriander seeds with the allspice berries over moderate heat until fragrant and lightly browned, about 1 1/2 minutes. Transfer to a spice grinder and let cool completely; grind to a powder. Stir in the sugar and 1 1/2 teaspoons of salt. Rub the mixture all over the roast and let stand at room temperature for 30 minutes, turning occasionally.
Preheat the oven to 400°. In an ovenproof skillet just large enough to hold the roast, heat the vegetable oil until shimmering. Add the beef; cook over moderate heat until browned all over, about 3 minutes per side. Transfer the skillet to the oven. Cook the roast, turning a few times, for about 30 minutes, until an instant-read thermometer inserted in the center registers 120° for rare. Transfer the roast to a carving board to rest for 15 minutes.
Thinly carve the roast beef across the grain and arrange on a platter. Garnish with the watercress and serve with the tomato salsa.