Cut the tomatoes lengthwise in half; remove the seeds and membranes. Place the tomatoes cut-side-down, on a platter; let drain for 15 minutes. In a large bowl, mix the pepper, salt, and rosemary. Add 6 tablespoons olive oil and garlic and whisk to blend. Add the tomato halves and toss to coat. Let them marinate for 15 minutes.
Position a rack in the center of the oven and preheat to 375°F. Line a rimmed baking sheet with parchment paper.
Arrange the tomato halves, cut side up, on the prepared baking sheet. Drizzle any oil mixture remaining in the bowl over the tomatoes. Roast until the tomatoes are slightly softened and browned on the bottoms and around the edges, about 65 minutes.
Arrange the tomatoes on a platter and sprinkle with the crumbled cheese. Tuck watercress in between the tomatoes. Serve warm or at room temperature.
Notes
Make ahead tip: The tomatoes can be roasted 3 hours ahead. Cover the tomatoes loosely and let stand at room temperature.