Toast mustard and cumin seeds and cool.
Pound garlic, jalapeno (to taste), and ginger to a paste with 1 teaspoon salt using a mortar and pestle (or mince and mash with a large heavy knife), then stir in curry powder, coriander, and mustard and cumin seeds.
Heat butter in a 6-quart heavy pot over moderately high heat until hot but not smoking, then saute onion, stirring, until golden, about 8 minutes. Add curry paste and cook over moderately low heat, stirring, 2 minutes.
Add zucchini and cook, stirring, until it begins to appear moist, 3 to 5 minutes. Add coconut milk and remaining teaspoon salt and bring to a boil, then reduce heat and simmer, covered, stirring occasionally, until zucchini is just tender, 10 to 12 minutes.
Serve sprinkled with cilantro and slivered almonds.
Makes 6 main-course or 8 side-dish servings.
Accompaniment: cooked basmati rice.