1) Place all vegetables in a large mixing bowl. Add 4 tbsp oil and season with salt and pepper, to taste. Toss well making sure each vegetable gets covered with oil. Place a griddle pan over medium heat. Once heated, place your vegetables on griddle and cook accordingly: griddle mushrooms about 5 mins per side; peppers and squash about 3 mins per side.
2) Once all the vegetables are cooked place them in a large serving bowl to slightly cool. Dice and add feta cheese, pitted olives, spinach and basil leaves. Gently toss to mix well and drizzle more oil, to taste, if necessary and serve.