1. Make the dressing: Mince garlic on a cutting board and sprinkle heavily with salt; using a knife, scrape garlic and salt together to form a smooth paste. Transfer paste to a bowl and whisk in oil, juice, mustard, shallot, and salt and pepper; set aside.
2. Make the salad: Arrange all ingredients in separate rows on a large serving platter; drizzle dressing over all ingredients, season with salt and pepper, and garnish with basil and scallions just before serving.