In a soup pot, warm a small amount of the vegetable broth over medium-high heat. Add the onion, carrots, and cayenne pepper, if using. Sauté until the onion and carrots begin to soften. Add the garlic and ginger and cook, stirring, until fragrant.
Add the tomato paste, lemon juice, curry powder, turmeric, and black pepper. Add a splash more vegetable broth as needed to prevent sticking. Cook, stirring, for a few minutes.
Add the remaining vegetable broth and bring to a boil. Add the brown basmati rice and return to a boil. Reduce the heat to medium-low and simmer for 15 minutes.
Add the red lentils and thyme leaves. Continue simmering for another 15 minutes, or until the rice and lentils are tender. Add additional broth or water as needed to reach your desired consistency.
Taste and adjust the seasoning as needed. Ladle into bowls, serve, and enjoy!