Heat oil in instant pot on "sauté" setting. Add onion and celery and cook until softened, about 5 minutes.
Stir in garlic, tomato paste, cumin and thyme and cook for one more minute.
Stir in water, scraping any browned bits off the bottom of the instant pot. Switch off instant pot.
Stir in broth, diced tomatoes, carrots and lentils. Season with salt and pepper, add bay leaves if using.
Close lid, setting valve to "sealing" position. Cook on manual, high pressure for 16 minutes (12 minutes for quicker-cooking lentils like brown ones). Allow 5 minutes of natural pressure release, then release remaining pressure manually.
Open instant pot and stir through spinach. Serve with lemon juice, chopped parsley and parmesan cheese to taste.
Lentils: French or green lentils stay firmer and hold their shape better. For brown lentils, pressure cook for 12 minutes – they do turn out very soft otherwise.
Broth: Vegetable or chicken broth works best.
Carrots: Dice chunkier if you prefer firmer carrots in your soup, dice smaller to cook them to fully softened.
Feel free to add chopped bacon or crumbled sausage with the onion, or stir in diced (fully cooked!) ham or diced smoked tofu in with the spinach at the end.
Cooking time: Always make sure to try the lentils before serving your soup, to make sure they are fully cooked. If they still have a raw taste to them, lock your instant pot lid again and pressure cook for 1-2 more minutes.
Freezer tips: This soup can easily be frozen (best with French or green lentils). Let the soup cool, then freeze in freezer friendly containers. Defrost in the fridge overnight, then gently reheat in a pot on the stove with a splash of broth added until steaming hot all the way through.
Stovetop instructions: Make the soup in a large pot or Dutch oven on the stove (just follow all of the same instructions, in a pot on the stove). Instead of pressure cooking, simmer the soup with the lid closed. The simmer time will again depend on your lentils – I recommend planning for 30-45 minutes, just check the package of your lentils for an indicated time and use it as your guide.