Cut the zucchini and summer squash into medium-thick slices. Lightly brush them with a little oil and grill on both sides until gently browned and just soft.
While the zucchini and squash are grilling, whisk together the lemon juice and olive oil, along with a little black pepper, in a bowl that will be big enough for the vegetables. Shred the basil.
Once the zucchini and squash are ready, carefully toss them in the lemon and oil mixture (whisk a little more just before adding if they have separated), along with the shredded basil. Leave to marinade for a good 5 minutes and serve, or else leave them to come to room temperature and refrigerate until you need them. They should keep fine a day or two refrigerated. Best brought up to room temperature before eating, though chilled also works.
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Notes
These would be great to nibble on alongside other picnic food, or else as a side to eg grilled fish, steak or chicken.If you don't have yellow summer squash, you can also simply use additional zucchini.