Heat the olive oil in a large pot or dutch oven over medium heat.
Saute the onion until soft and slightly translucent.
Add the garlic and saute 30 seconds longer.
Next add the carrots and celery and saute for about 2 minutes more until softened.
Add the vegetable broth and potatoes to the pot. Bring to a low boil and simmer for about 5 minutes until the potatoes are starting to get tender but are not all the way cooked when pierced with a fork.
Stir in the red lentils and kale. Cook for 10-15 minutes until the lentils and kale are tender and the potatoes are cooked through.
Taste and add salt and pepper as needed. Stir in the lemon juice and serve.
Leftover soup can be refrigerated for up to 3 days or frozen for up to 6 months. Add some extra water or vegetable broth when reheating if the soup has gotten too thick.