Combine broth and lentils in a saucepan and bring to a boil. Reduce heat and simmer until lentils are soft and broth is absorbed, 20 to 30 minutes. Drain extra broth, if necessary. Mash lentils slightly and transfer to a large mixing bowl.
Preheat the oven to 350 degrees F (175 degrees C). Grease a loaf pan.
Add oats, Cheddar cheese, panko, eggs, onion, ketchup, garlic, parsley, Worcestershire sauce, basil, red pepper flakes, salt, and black pepper to the bowl with the lentils and mix well. Transfer mixture to the prepared loaf pan.
Bake in the preheated oven until loaf starts to brown, 35 to 45 minutes. Let sit 10 minutes before serving. Loosen sides using a spatula before slicing.