In a large heavy-bottom pot over high heat, combine chicken stock, garlic, ginger, curry paste, coriander, cumin, peppercorns and cilantro. Bring to a boil; reduce heat and simmer 30 minutes.
Strain broth over a bowl through a fine-mesh sieve; discard solids.
Return broth to pot; add coconut milk, shallots, mushrooms and chicken.
Return to a boil, then reduce heat to a simmer and cook until chicken is cooked through, 5 to 7 minutes.
In a small bowl, combine fish sauce, lime juice, brown sugar and jalapeño, if desired; stir until sugar is dissolved, then add mixture to soup.
Garnish with tomato, coconut shavings and cilantro. Serve hot.