Heat oven to 350 degrees F. Line an 8x4 inch loaf pan with aluminum foil; spray foil with nonstick cooking spray.

In a food processor, process onion and carrot until finely chopped. Heat oil in a medium skillet over medium heat. Cook onion and carrot until softened, about 5 minutes. Place parsley and dill in processor; process until chopped. Add white fish, salmon, eggs, salt, and pepper; process in pulses until fairly smooth. Add vegetables; pulse to combine. Gradually add water and process until incorporated. Transfer mixture to loaf pan; smooth top.

Cover with aluminum foil. Place a roasting pan, half full with hot water, in oven. Place loaf pan in roasting pan (water should come at least halfway up sides of pan). Bake 1 hour until terrine is set. Remove from oven. Cool slightly before refrigerating. When chilled, unmold and cut into slices.