Prepare Chicken: Shred cooked chicken breast. Use poached chicken or shredded rotisserie chicken.
Prepare Lemongrass: Hit the lemongrass with the back of a knife to release its flavor.
Sauté Veggies: Heat butter and oil in a large pot. Sauté mushrooms and red pepper for a few minutes, then add ginger and garlic. Stir in curry paste and toast for 10 seconds.
Build the Soup: Deglaze the pan with chicken stock, then add coconut milk, chicken, lime juice/zest, fish sauce, lemongrass, and scallions. Simmer uncovered for 15 minutes.
Add Zucchini: Stir in zucchini and cook for 5 more minutes, or until tender.
Serve: Remove lemongrass, garnish with cilantro, and serve with a squeeze of lime.