Heat the olive oil in a large pot over medium heat. Add the onion, carrots, and red bell pepper. Cook for 5 minutes until the vegetables begin to soften.
Stir in the garlic, curry powder, and turmeric. Cook for 1 minute until the spices smell fragrant.
Add the rinsed lentils, diced tomatoes, and vegetable broth. Bring the mixture to a boil, then reduce to a simmer. Cover and cook for 25 minutes until the lentils are tender.
While the curry simmers, heat a separate skillet over medium heat. Add the cauliflower rice and cook for 5 to 7 minutes, stirring occasionally, until it is just tender. Season lightly with salt and pepper.
Once the lentils are done, stir the spinach into the curry and cook for 2 minutes until wilted. Taste and adjust salt and pepper as needed.
Divide the cauliflower rice among four bowls. Spoon the curry over the top and sprinkle with cilantro if using.