Heat the olive oil in a large skillet over medium high heat. Add the ground sirloin, breaking up with a spoon while it begins to brown. Add the vegetables and salt, cook stirring frequently for 7-9 minutes. Add the ground cumin and cook for about 3 more minutes.
Add the crushed tomatoes, tomato paste, chipotle peppers, liquid smoke, and water to the pan. Bring the mixture to a slow simmer and cook for 5 more minutes, stirring often, until the mixture has thickened and meat is fully cooked through.
Stir in the agave nectar, check seasonings, and adjust if necessary.
Toast the hamburger buns.
Top each bottom bun with one-fourth of the sloppy joe mixture and finish with the top bun.