Using sauté feature of Instant Pot, heat the pot and add onion, dry sauté, stirring occasionally as the onion lightly browns, adding a few tablespoons of stock to prevent sticking and to create a light brown caramelization, about 4-5 minutes. Add the garlic and stir for about two minutes. Stir in the carrots and cook another 3-4 minutes. Stir in the potatoes, lentils, seasoning and stock, and tomatoes (if using). Add salt, if using, now or later.
Lock the lid in place and set the pressure to 8 minutes. Let the pressure come down naturally, or release pressure at 25 minutes. Carefully remove the lid.
Discard the bay leaf. Add the salt, pepper, and lemon juice. If using the spinach, add to the soup now, it let it sit a few minutes to wilt. Serve garnished with chopped parsley, cilantro, and or vegan sour cream if desired.