Preparation Place peas in a large pot; add cold water to cover by 2". Let soak overnight. Drain peas; return to pot. Add onion, carrot, celery, and bacon. Add water to cover ingredients by 2". Bring to a boil over medium-low heat, stirring often. Continue cooking at a very low simmer, stirring every 15 minutes and occasionally skimming foam from the surface, until peas are almost tender, 2-3 hours (time will vary depending on age of peas). Season with salt. Simmer until peas are very tender, 15-45 minutes longer. Season to taste with salt and pepper. DO AHEAD: Can be made 2 days ahead. Let cool slightly. Chill uncovered until cold, then cover and chill. Return to a simmer before continuing. Season foie gras cubes, if using, with salt; divide among warmed bowls. Ladle hot soup over cubes and let stand until foie gras is warmed through, about 2 minutes. Drizzle with oil; sprinkle with pepper.
Preparation Place peas in a large pot; add cold water to cover by 2". Let soak overnight. Drain peas; return to pot. Add onion, carrot, celery, and bacon. Add water to cover ingredients by 2". Bring to a boil over medium-low heat, stirring often. Continue cooking at a very low simmer, stirring every 15 minutes and occasionally skimming foam from the surface, until peas are almost tender, 2-3 hours (time will vary depending on age of peas). Season with salt. Simmer until peas are very tender, 15-45 minutes longer. Season to taste with salt and pepper. DO AHEAD: Can be made 2 days ahead. Let cool slightly. Chill uncovered until cold, then cover and chill. Return to a simmer before continuing. Season foie gras cubes, if using, with salt; divide among warmed bowls. Ladle hot soup over cubes and let stand until foie gras is warmed through, about 2 minutes. Drizzle with oil; sprinkle with pepper.