Add vegetable broth and lentils to a medium sized pot and bring to a boil on medium-high heat.
Reduce heat to low, and cook uncovered, for 18-20 minutes until tender then remove from heat and drain.
Add vegetable oil, onions and bell pepper to a large skillet on medium heat and cook, stirring occasionally, for 6-8 minutes, until translucent.
Add in garlic, salt and pepper and stir well, cooking for 1 minute until garlic is fragrant.
Add in tomato sauce, tomato paste and water, whisk well until the tomato paste is fully combined.
Add in the dark brown sugar, cayenne pepper, ground mustard and paprika and stir to combine.
Add the lentils into the skillet, stir and cook for 6-8 minutes until the sauce is thickened and the lentils are well coated.