Black-eyed Pea and Mustard Green Soup

Ingredients

• 8 ounces dried field peas, such as lady peas, crowder peas or black-eyed peas, soaked overnight, rinsed and drained
• 1 tablespoon olive oil
• 1 small yellow or white onion (6 ounces), thinly sliced
• 2 medium carrots (10 ounces total), scrubbed and diced
• 2 celery stalks, diced
• 5 cloves garlic, thinly sliced
• 6 cups vegetable broth
• 1 small bunch mustard greens (4 ounces), stemmed and chopped
• 1 tablespoon balsamic vinegar, plus more to taste
• 1 tablespoon chopped fresh rosemary
• 1 teaspoon fine sea salt, plus more to taste
• 1/2 teaspoon crushed red pepper flakes, plus more to taste