To make ahead: Cover and refrigerate for up to 3 days.
Look for prepared green curry paste--a fiery mixture of green chiles and Thai seasonings--in the Asian section of large supermarkets. The heat and salt level can vary widely depending on brand. Be sure to taste as you go; if you like your curry very spicy, you can turn up the heat by using a little more fresh serrano chile.
Look for lemongrass--a woody, scallion-shaped herb with an aromatic lemon flavor--in the produce department of well-stocked supermarkets. To use, trim off the root end and grassy top. Peel off the woody outer leaves. Thinly slice the softer inner stalk, then finely chop.
2 vegetable, 1 fat