Preheat the oven to 220°C (425°F). Line a large baking sheet with parchment paper to prevent sticking and promote even roasting. Using a large tray is important so the vegetables can spread out in a single layer. Overcrowding the tray will cause the vegetables to steam rather than roast, preventing proper caramelization.

Wash all vegetables thoroughly and pat them dry. Slice the zucchini into 2 cm chunks, ensuring pieces are similar in size so they cook evenly. Cut the red and yellow bell peppers into 2 cm squares, removing seeds and inner membranes. Halve the cherry tomatoes and cut the red onion into small wedges about the same size as the other vegetables. Uniform cutting ensures even roasting and balanced texture.

Place the prepared vegetables in a large mixing bowl. Drizzle with 2 tablespoons olive oil and season with salt and black pepper. Toss thoroughly until every piece is lightly coated with oil and seasoning. The oil helps the vegetables caramelize while roasting and prevents them from drying out.

Spread the vegetables evenly across the prepared baking sheet in a single layer. Ensure the vegetables are not stacked on top of one another. Proper spacing allows hot air to circulate around each piece, helping the vegetables develop caramelized edges and concentrated flavor.

Place the tray in the preheated oven and roast for 25 minutes. About halfway through the cooking time, use a spatula to gently turn the vegetables so they brown evenly on all sides. The vegetables should become tender with lightly browned edges and slightly blistered tomatoes.

While the vegetables roast, prepare the dressing. In a small bowl combine the remaining 1 tablespoon olive oil, lemon juice, Dijon mustard, and minced garlic. Whisk vigorously until the dressing becomes smooth and slightly emulsified. Taste and adjust seasoning with salt and pepper if needed. The dressing should be bright, slightly tangy, and lightly garlicky.

Rinse the baby arugula under cold water and dry it thoroughly using a salad spinner or clean towel. Place the arugula evenly across a large serving platter or shallow bowl. The peppery flavor of arugula provides contrast to the sweetness of the roasted vegetables.

Once the vegetables are fully roasted, remove the tray from the oven and allow them to cool slightly for 2–3 minutes. Spoon the warm vegetables over the bed of arugula so the heat lightly softens the greens without wilting them completely.

Scatter the torn basil leaves evenly across the salad. The basil adds aromatic freshness that brightens the roasted vegetable flavors.

Drizzle the prepared lemon dressing evenly over the salad. Serve immediately while the vegetables are still warm and the arugula retains its crisp texture.