Quinoa Tabbouleh with Grilled Vegetables

Name: Quinoa Tabbouleh with Grilled Vegetables
Ingredients: 1 medium yellow squash (halved lengthwise); 1 medium zucchini (halved lengthwise); 1 large red bell pepper (quartered with seeds and membranes removed); 2 tablespoons olive oil PLUS 1/4 cup; Salt and freshly ground black pepper (to taste); 3 1/2 to 4 cups cooked quinoa (cooled and chilled); 1 bunch scallions (finely chopped); 3 large plum tomatoes (seeded and chopped); 3/4 cup chopped fresh mint; 1 small bunch parsley (finely chopped (approximately 3/4 cup), plus more for garnish); 4 cloves garlic (minced); 2 large lemons (juiced plus lemon slices for garnish)