Preheat the oven to 400°F (205°C).
Line a large baking sheet with parchment paper.
Chop 1 red bell pepper and 1 zucchini into bite-sized pieces; thinly slice 0.5 red onion.
On the prepared baking sheet, toss the red bell pepper, zucchini, 1 cup broccoli florets, 1 pint cherry tomatoes, and sliced red onion with 2 tablespoons olive oil, 3 cloves minced garlic, 1 teaspoon dried Italian seasoning, 0.5 teaspoon salt, and 0.25 teaspoon black pepper; spread in a single layer.
Roast the vegetables for 20–25 minutes, until tender and slightly caramelized.
Meanwhile, bring a large pot of salted water to a rolling boil and cook 12 ounces whole wheat spaghetti until al dente, according to package directions; reserve 0.5 cup pasta cooking water before draining.
Drain the spaghetti and transfer it to a large bowl.
Add the roasted vegetables to the bowl with the spaghetti.
Squeeze the juice of 1 fresh lemon over the pasta and vegetables and toss gently; add 1–2 tablespoons reserved pasta water if the mixture seems dry.
Taste and adjust seasoning as needed.
Serve immediately, garnished with chopped fresh basil or parsley (optional).