Heat a large skillet over medium-high heat and add the ground beef, diced yellow onion, and diced green bell pepper.
Cook until the beef is fully browned and the vegetables have softened, using a spatula to break the meat into small crumbles.
Stir in the tomato puree, smoked paprika, garlic powder, yellow mustard, maple syrup, sea salt, and black pepper until well combined.
Reduce the heat to low and allow the mixture to simmer for 10 minutes until the sauce has thickened and flavors have melded.
While the meat is simmering, heat the olive oil in a separate small non-stick pan over medium heat.
Add the thinly sliced red onions to the small pan and sauté, stirring frequently, until they are dark golden brown and crispy.
Lightly toast the whole wheat slider buns in a toaster or under a broiler until the edges are crisp.
Spoon the smoky beef mixture generously onto the bottom halves of the toasted slider buns.
Top the beef with the crispy red onions and place the top bun halves over the onions.
Serve immediately while the sliders are hot and the onions are crunchy.