1. Place the frozen peas in a medium saucepan, and just cover them with stock.
2. Add butter, tarragon, and chervil (or Italian parsley), and place the pan over high heat.
3. Bring just to a boil, and remove from heat.
4. Put one-third of the spinach into a blender and add one-third of the pea mixture.
5. Be sure to put the lid on the blender and to hold on with a dish towel, as processing hot ingredients in a blender can cause the lid to pop off.
6. Puree on low speed, then high, being careful to hold the lid in place.
7. Repeat until all the spinach and pea mixture is combined and smooth. Season to taste.
8. Spoon onto plate when serving cod.
Adapted from chef Daniel Hover of Ritz Seafood in Voorhees
Per serving (based on 8): 154 calories, 5 grams protein, 7 grams carbohydrates, 2 grams sugar, 12 grams fat, 31 milligrams cholesterol, 398 milligrams sodium, 3 grams dietary fiber.