Place all vegetables in a large mixing bowl. Add 1/4 cup oil and season with salt and pepper, to taste. Toss well making sure each vegetable gets covered with oil. Place a grill pan over medium heat. Once heated, place your vegetables on grill and cook accordingly: grill mushrooms about 5 minutes per side; peppers and squash about 3 minutes per side.
Once all the vegetables are grilled 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.