In a large saucepan over medium-high heat, heat the oil until it shimmers. Add the onion and cook, stirring frequently, until softened, 6 minutes. Add the carrots, celery and garlic and cook, stirring occasionally, until the onion starts to turn golden, 8 to 9 minutes.
Add the soaked peas and vegetable broth and bring to a boil. Reduce the heat so the liquid is at a simmer, cover and cook until the peas are just beginning to soften, 12 to 15 minutes. Stir in the greens, vinegar, rosemary, salt and red pepper flakes, re-cover, and cook until the peas and greens are tender, 10 to 12 minutes.
Taste, and season with more vinegar, salt and/or red pepper flakes as needed. Divide among serving bowls and serve hot.
