How to Make It
Preheat oven to 425°. Drain tuna, reserving oil; set aside.
Combine squash, potatoes, radishes, 1 1/2 tablespoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a large bowl; toss to coat. Spread squash mixture in a single layer on a rimmed baking sheet. Add green beans, tomatoes, 1/2 tablespoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper to bowl; toss to coat. Arrange tomatoes on a second rimmed baking sheet; reserve green beans. Roast squash mixture and tomatoes 15 minutes. Add green beans to pan with tomatoes; stir squash mixture. Roast 15 more minutes or until beans are crisp-tender and potatoes are cooked through. Let vegetables cool to room temperature.
Whisk together garlic and next 4 ingredients in a medium bowl. Combine 1/4 cup reserved oil from canned tuna with remaining 1/4 cup olive oil. Add oil to vinegar mixture in a slow, steady stream, whisking constantly until smooth. Sprinkle with remaining 1/2 teaspoon salt and remaining 1/4 teaspoon pepper.
Combine watercress and half of the vinaigrette in a large bowl; toss to coat. Divide watercress mixture evenly among 4 plates. Top evenly with roasted vegetables. Flake tuna evenly over vegetables; drizzle evenly with remaining vinaigrette.