1/3 cup coarsely chopped peeled fresh ginger
4 garlic cloves, thinly sliced
3 tablespoons water
3 kaffir lime leaves
1 fresh lemongrass stalk, tough outer leaves removed, 4 inches of thick end thinly sliced
1/2 to 1 small whole dried red chile (such as chile de árbol)
3 tablespoons peanut oil, divided
24 large uncooked shrimp, peeled, deveined, cut in half lengthwise, shells reserved
1 large red onion, halved through core, thinly sliced, divided
2 tablespoons fish sauce (such as nam pla or nuoc nam)
1 rounded tablespoon chopped palm sugar or dark brown sugar
4 cups canned unsweetened coconut milk
2 1/2 cups vegetable broth
10 ounces shiitake mushrooms, stemmed, stems reserved, caps sliced
9 ounces dried flat rice noodles
4 garlic cloves, thinly sliced
1 tablespoon toasted sesame oil (such as Asian)
1/2 cup skinless peanuts, toasted, coarsely chopped
1/4 cup fresh lime juice
1/2 cup thinly sliced green onions
1/3 cup fresh cilantro leaves
1 lime, cut into 6 wedges