In a skillet, heat the olive oil over medium heat. Add the onion and cook for 5 minutes, or until softened.
Add the garlic, paprika, cumin, cayenne pepper (if using), salt, and black pepper. Cook for 2-3 minutes, or until fragrant.
Add the lentils to the skillet and stir to combine.
In a separate bowl, mix together the vegetable broth, passata, tomato puree, agave syrup, and apple cider vinegar. Pour the mixture into the skillet and stir to combine.
Cook the mixture for 5-10 minutes, or until heated through and the sauce has thickened.
Toast the hamburger buns and serve the lentil mixture on top. Top with pickles and coleslaw, if desired