Grilled Summer Squash with Roasted Pistachio Sauce

Name: Grilled Summer Squash with Roasted Pistachio Sauce
Ingredients: 5 medium scallions , root ends trimmed; 3 medium tomatillos (about 6 ounces), husks removed, tomatillos rinsed; 2 medium serrano chiles , stemmed; 8 garlic cloves (unpeeled), plus 1/4 teaspoon grated garlic, divided; 1 cup unsalted dry-roasted pistachios , plus chopped pistachios, for garnish; 1 cup chopped fresh cilantro leaves and tender stems, plus whole cilantro leaves, for garnish; 2 1/4 teaspoons kosher salt , divided; 1 1/2 pounds medium-size mixed summer squash (about 3 squash), halved lengthwise and cut crosswise into 3-inch pieces; 3 tablespoons olive oil , divided, plus more for drizzling; 2 tablespoons fresh lime juice; Flaky sea salt , for garnish