For The Roasted Pistachio Sauce

1. Heat a large cast-iron or heavy skillet over medium-high just until smoking.
2. Add scallions, tomatillos, chilies, and unpeeled garlic cloves*; cook, flipping occasionally until mixture is charred in spots and slightly softened, 4 to 6 minutes for scallions, 8 to 10 minutes for chiles and garlic*, and 10 to 12 minutes for tomatillos.
3. Transfer to a cutting board, and let cool for 5 minutes.
4. Coarsely chop scallions, tomatillos, and chiles. Remove and discard garlic skins, and add garlic to scallion mixture; set mixture aside.
5. Process whole pistachios in a food processor until very finely chopped, about 30 seconds, stopping to scrape down sides of processor bowl after 15 seconds.
6. Add charred scallion mixture, chopped cilantro, and 1 teaspoon kosher salt. Process until mostly smooth and creamy, about 1 minute and 30 seconds, stopping to scrape down bowl after 45 seconds.
7. Transfer mixture to a small bowl, and press plastic wrap directly on the surface of the sauce. Set aside at room temperature until ready to use.

For the Grilled Summer Squash

1. Preheat grill to high (450°F to 500°F).
2. Toss together squash, 1 tablespoon oil, and remaining 11/4 teaspoons kosher salt in a large bowl.
3. Place squash, cut sides down, on oiled grates; grill, uncovered, until deeply charred, 5 to 7 minutes.
4. Flip squash, and grill just until squash is tender, 1 to 2 minutes.
5. Transfer to a small baking dish.
6. Stir together lime juice, grated garlic*, and remaining 2 tablespoons oil in a small bowl.
7. Pour mixture over squash, and gently toss to combine; let marinate 15 minute

To Finish

1. Spread 1 cup pistachio sauce on a large platter.
2. Top with grilled squash, and spoon any remaining marinade in baking dish over squash.
3. Garnish with chopped pistachios and cilantro leaves. Drizzle with additional oil, and garnish with flaky salt.
4. Serve remaining pistachio sauce on the side, or reserve for another use