Finely dice the yellow onion and green bell pepper, and mince the garlic clove.
Heat the avocado oil in a medium non-stick skillet over medium-high heat.
Add the onion and bell pepper to the skillet, sautéing for 3-4 minutes until softened and translucent.
Add the ground beef and minced garlic to the pan, breaking the meat apart with a spatula until fully browned.
Stir in the tomato puree, tomato paste, coconut aminos, yellow mustard, chili powder, smoked paprika, sea salt, and black pepper.
Reduce the heat to low and let the mixture simmer for 5-7 minutes, stirring occasionally until the sauce has thickened and glossy.
While the beef simmers, lightly toast the whole wheat slider buns in a toaster or under a broiler until golden.
Spoon the sloppy joe mixture evenly onto the bottom halves of the toasted buns, top with the other halves, and serve immediately.