Prep ingredients: Preheat oven to 450°F with a rack in the upper third. Halve, peel, and cut all of the onion into ½-inch thick wedges through the root end. Trim ends from zucchini, then halve lengthwise and cut into ½-inch thick half-moons. Finely grate ¼ teaspoon lemon zest into a small bowl, then cut lemon into quarters.
Roast vegetables: Transfer onions, zucchini, grape tomatoes, and 2 of the lemon quarters to a rimmed baking sheet (save remaining lemon for your own use); toss with 1 tablespoon oil and season with salt and pepper. Roast on upper oven rack until vegetables just tender, 10–12 minutes.
Prep salmon: Peel and finely chop 1 teaspoon garlic. Pat salmon dry; season with salt and pepper. Into the small bowl with lemon zest, add half of the chopped garlic and 1 teaspoon oil, and stir to combine. Season with a pinch each of salt and pepper.
Roast salmon: Transfer remaining chopped garlic to baking sheet, then stir with vegetables to combine. Transfer salmon, skin-side down, to the baking sheet, then spread lemon-garlic oil over top. Roast on upper oven rack until salmon is cooked through and opaque, about 8 minutes.
Prep basil & olives: Meanwhile, coarsely chop olives, removing any pits, if necessary. Pick and coarsely chop basil leaves, discarding stems.
Finish & serve: Transfer olives and basil to the baking sheet, stirring to combine with the vegetables. Using tongs, squeeze the roasted lemon wedges over the vegetables; season to taste with salt and pepper. Drizzle salmon and vegetables with a little olive oil, and serve.