In a medium saucepan or dutch oven, heat up the olive oil. Melt in the onions, diced, until soft and translucent.
Add red lentils and cover with broth or water. Bring to a boil, cover tightly, reduce heat and simmer until they are tender, about 15 minutes, stirring regularly. Pour crushed tomatoes (or tomato sauce) and stir well.
Add cherry tomatoes, fresh baby spinach and give a good stir. Season with salt and freshly ground pepper, add some chili flakes, and serve with a side of rice.