Cook the brown rice according to package directions and set it aside once it is tender.
Rinse the red lentils under cold water until the water runs clear.
Heat a large pot over medium heat. Add the diced onion and cook for 4 minutes until it softens.
Stir in the minced garlic and grated ginger. Cook for 1 minute until fragrant.
Add the curry powder, cumin, and turmeric. Stir for 30 seconds to toast the spices.
Pour in the diced tomatoes with their juices, the rinsed lentils, and the vegetable broth. Season with salt and pepper.
Bring the mixture to a simmer, then reduce the heat and cook uncovered for 20 minutes, stirring occasionally, until the lentils are soft.
Stir in the fresh spinach and cook for 2 minutes until it wilts into the curry.
Spoon the curry over the cooked brown rice and top with chopped cilantro if using.