Adjust the oven rack to the middle position and heat the oven to 425°F. Toss the fennel and onions with oil, salt, and pepper on a rimmed baking sheet. Roast until the vegetables are tender and lightly browned, 15 to 20 minutes, stirring halfway through for even caramelization.

Bring a large pot of water to a boil. Add the pasta and salt and cook, stirring often, until al dente. Drain the pasta and transfer it to a large bowl while still steaming to help it absorb the dressing later.

Whisk the lemon zest, lemon juice, garlic, and salt together in a small bowl. Whisking constantly, slowly drizzle in the remaining oil until the dressing is fully emulsified and thickened.

Stir the dressing, roasted vegetables, and tomatoes into the hot pasta and let the mixture cool to room temperature, about 30 minutes. This resting period is vital—it allows the pasta to soak up the vinaigrette while it is still warm.

Stir in the basil and season with salt and pepper to taste. Serve at room temperature, once the pasta has fully absorbed the bright dressing.