- Preheat oven to 450°F.
- Place asparagus in a large bowl. Add tomatoes and oil and toss to coat. Spread in a heavy roasting pan or rimmed baking sheet, spooning the tomatoes between and on top of the asparagus. Sprinkle with 1/2 teaspoon salt and add a generous grinding of pepper. Roast until the asparagus is crisp-tender and the tomatoes are warmed and slightly crinkled, about 15 minutes. Set aside until ready to serve.
- Whisk lemon juice, orange juice, honey, mustard and remaining 1/4 teaspoon salt in a medium bowl until blended. Reserve half of the dressing in a small bowl.
- Add watercress to the medium bowl; toss to coat. Spread the watercress on a platter. Arrange the roasted asparagus on the watercress and top with tomatoes. Drizzle the reserved dressing over the asparagus and tomatoes; sprinkle with dill. Serve warm or at room temperature.
To Make Ahead: Prepare through Step 2 up to 2 hours ahead. Cover and refrigerate the dressing (Step 3) for up to 5 days.