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 40
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.
