Spicy Thai-Flavor Soup

Reserved shells from shrimp, see below

6 scallions

4 cloves garlic

1 stalk lemongrass, lightly smashed, or zest of

1 lemon

1 tablespoon whole black peppercorns

1 (2-inch) piece fresh ginger , sliced thick

4 kaffir lime leaves, fresh or frozen, or juice and skin of

1 lime

2 serrano chiles , halved lengthwise

1 recipe Asian Chicken Stock

1 (15-ounce) can straw mushrooms , drained and rinsed

2 limes, juice and skins

4 serrano chiles, halved lengthwise

1 teaspoon fish sauce, or more to taste

5 kaffir lime leaves, fresh or frozen, optional

Kosher salt

1 pound large shrimp, shelled and sliced in

1/2 lengthwise

2 tablespoons chopped fresh cilantro leaves, plus more for garnish

1 tablespoon Thai basil leaves or regular basil leaves