Heat olive oil in a large pot over medium heat. Add the onion, carrots, and celery with a pinch of salt and cook for 5–7 minutes, until softened.
Stir in the garlic and tomato paste. Cook for 1–2 minutes, stirring constantly, until the tomato paste darkens slightly and smells sweet.
Add the potatoes, diced tomatoes, oregano, thyme, bay leaf, and vegetable broth. Stir, bring to a boil, then reduce the heat and simmer for 15–20 minutes, or until the potatoes are almost tender.
Stir in the green beans, peas, and corn. Simmer for another 5–10 minutes, until all the vegetables are cooked through but not mushy.
Add the spinach or kale and cook for 1–2 minutes, just until wilted. If you’re using beans or lentils, stir them in now and heat through.
Remove the bay leaf. Stir in lemon juice, taste, and season with more salt, pepper, and red pepper flakes if you like.
Serve hot, garnished with fresh herbs and an extra squeeze of lemon.