Add a large stockpot or dutch oven over medium heat.
Add the olive oil and heat until it becomes hot, about 1 minute.
Add the onion and cook, stirring occasionally, until it is soft and translucent, about 3 minutes.
Add bell pepper, garlic, and tomato paste. Stir and cook for an additional minute.
Next, add the vegetable stock and stir until the tomato paste has dissolved.
Add the diced tomatoes, lentils, chili powder, cumin, salt, paprika, oregano, and red pepper flakes.
Bring the soup to a boil and then reduce the heat to low and simmer for 10 minutes. If your soup begins to look too dry, feel free to add more vegetable stock or water as needed.
Add the red kidney beans and great northern beans. Simmer for another 15 minutes, or until the lentils are tender.
Serve with your favorite toppings and store any leftover chili in an airtight container in the fridge for up to three days.