Preparation

Make the curry paste

• Combine the lemongrass, chiles, oil, shallot, garlic, ginger, cilantro, coriander, cumin, and lime zest in a blender. Add 3 Tbs. water, 1/2 tsp. salt, and 1/8 tsp. pepper, and blend to form a coarse, wet paste.

Make the curry

• In a 4-quart saucepan, cook the curry paste over medium heat, stirring frequently, until much drier and just beginning to stick to the pan, about 4 minutes. Add the coconut milk, broth, 1/2 cup water, and the sugar. Increase the heat to medium high, bring to a boil, reduce to a simmer, and cook, stirring occasionally, for 5 minutes. Add the carrots and snap peas, bring to a simmer, and cook until just barely tender, about 4 minutes. Stir in the fish sauce, and season to taste with salt.
• Pat the scallops dry. Heat a large cast-iron or other heavy-duty skillet over medium-high heat until very hot. Add the remaining 1 Tbs. oil and the butter. Sprinkle the scallops all over with coriander and ¼ tsp. salt. Working in batches if necessary to keep from crowding, cook the scallops, undisturbed, until very well browned on one side, 2 to 3 minutes. Flip and cook until just barely cooked through, 2 to 4 minutes more, depending on size. Transfer to a plate. Repeat, if necessary, wiping out the pan and adding more butter and oil as needed.
• Stir the chopped herbs into the curry. Serve over rice or noodles, if you like. Top the curry with the scallops.

Make Ahead Tips

The curry paste can be stored covered and refrigerated, for up to 5 days.