Put the lentils in a small sauce pot and pour in 4 cups beer. Cover and bring to a boil. Once boiling, lower heat and simmer for about 20 minutes, until lentils are soft. Drain and set aside.
About 10 minutes before the lentils are done boiling, preheat a medium sauce pan pot over medium heat. Saute the onion and pepper in the oil (~ 7 minutes), until softened.
Add the garlic and saute a minute more.
Add the cooked lentils, and the the rest of the spices (except the maple syrup & dried mustard). Stir gently until just combined.
Add the tomato sauce and tomato paste. Cook for about 10 minutes.
Add the maple syrup and mustard and heat through.
Turn the heat off and let sit for about 10 minutes, so that the flavors can meld, or go ahead and eat immediately if you can’t wait. I like to serve these open faced, with a scoop of sloppy joe on each slice of the bun.