Preheat oven to 350 degrees F. Preheat grill to medium high. Toss the squash and zucchini with the olive oil and grill the rounds until golden brown about 3 minutes per side. Remove zucchini and squash rounds from grill and cool on a wire rack. Toss the zucchini and squash together with the basil, lemon juice, lemon zest, cheese, and salt. Transfer to a 1 1/2-quart casserole and bake until the squash and zucchini mixture is heated through about 15 to 20 minutes. Sprinkle the pine nuts over casserole and serve warm.