Add the red lentils and 3/4 cup water to the pot. Bring to a boil, then reduce to a simmer. Cook about 10 minutes, stirring occasionally, until the lentils are tender but not falling apart. Add tomato paste, mustard, maple syrup, apple cider vinegar, Worcestershire sauce, chile powder, and salt. Stir to combine. Simmer until the sauce thickens a bit, 3–5 minutes more. Serve on toasted bun with whatever toppings you have available.