Poached cod in tomato sauce with prunes is reminiscent of the Italian Renaissance, when cooks often combined sweet and savory flavors in a single dish.
Heat grapeseed oil in an extra-large straight-sided skillet (or 2 medium skillets) over medium heat. Add onion, and cook until translucent, about 6 minutes. Add vinegar, and cook for 30 seconds. Add tomato puree and water, and bring to a boil.
Season cod with salt and pepper. Add cod, prunes, and rosemary to sauce. Cover, and simmer until cod is just cooked through, 12 to 15 minutes. Serve warm, drizzled with olive oil and seasoned with pepper.