Set your Instant Pot to 'Sauté' mode. Once it’s hot, add the olive oil.
Add the diced onion and sauté for 3-4 minutes until translucent. Stir in the minced garlic and cook for an additional minute.
Add the diced carrots and celery, and sauté for another 3-4 minutes until slightly softened.
Stir in the ground cumin, smoked paprika, and dried thyme, allowing the spices to toast for about 1 minute.
Pour in the vegetable broth and add the rinsed lentils and diced tomatoes (with juices). Stir to combine.
Seal the Instant Pot lid and ensure the vent is closed. Set the Instant Pot to 'Manual' or 'Pressure Cook' mode for 15 minutes.
Once the cooking time is complete, allow the pressure to release naturally for 10 minutes, then carefully perform a quick release for any remaining pressure.
After removing the lid, stir in the chopped spinach or kale until wilted. Season with salt and pepper to taste.
Serve hot, and if desired, drizzle with lemon juice for added brightness.