Preheat your oven to 425°F (220°C). Line two large baking sheets with parchment paper to prevent sticking and make cleanup easier.
Prepare the vegetables. Cut all vegetables into similar-sized pieces so they roast evenly. Spread them out on the prepared baking sheets in a single layer, making sure not to overcrowd them.
Season the vegetables. Drizzle with olive oil, then sprinkle with salt, pepper, and oregano. Toss everything together with your hands to ensure even coating.
Roast the vegetables. Place the baking sheets in the preheated oven and roast for 25-30 minutes, stirring halfway through. The vegetables should be tender and slightly caramelized at the edges.
Make the dressing while vegetables roast. Whisk together olive oil, balsamic vinegar, lemon juice, minced garlic, and Dijon mustard in a small bowl. Season with salt and pepper to taste.
Prepare the salad base. While the vegetables are still roasting, arrange the mixed greens on a large serving platter or in a wide bowl.
Assemble the salad. Once the vegetables are done roasting, let them cool for about 5 minutes. Then arrange them over the greens while still slightly warm.
Add the finishing touches. Drizzle the dressing over the entire salad, then sprinkle with crumbled feta cheese and toasted nuts. Tear fresh basil leaves over the top for a pop of color and flavor.
Serve immediately or let it sit. This summer roasted vegetable salad can be served right away or allowed to sit for 10-15 minutes so the flavors can meld together beautifully.