In the same pan used for the buns, warm 1 to 2 tablespoons oil over medium-high heat until hot but not smoking. Add the finely chopped onion, season with salt and pepper, and cook, stirring occasionally, until the onion starts to soften, 2 to 4 minutes.
Add the lentils, green chiles, sloppy joe sauce base, 2 tablespoons [¼ cup] water, and up to 1 teaspoon [2 tsp] brown sugar (from your pantry) if using. Bring to a boil, then reduce to a simmer and cook, stirring occasionally, until the lentils are heated through and the sauce is thickened slightly, 2 to 3 minutes.
If you prefer a saucier filling, stir in more water, 1 tablespoon at a time, to reach the desired consistency. Remove from the heat and season to taste with salt and pepper.
While the sloppy joe filling is cooking, prepare the coleslaw.