Heat the vegetable oil in a skillet over medium heat.
Add the diced onions and sauté for 2–3 minutes, until softened and lightly golden.
Add the ground beef to the skillet, breaking it up with a spatula. Season with salt, black pepper, garlic powder, and paprika. Cook the beef, stirring occasionally, until fully browned and no longer pink
While the beef cooks, lightly toast the rolls under a broiler or in a toaster oven.
Once the beef is cooked through, form it into portions in the skillet, roughly the size of your rolls.
Place cheese over each pile. Let melt.
Use a spatula to chop the cheese into the meat, mixing slightly, then scoop each portion onto a toasted roll.
Spread mayonnaise on the top halves of the rolls.
Top the beef with shredded lettuce, tomato slices, and ketchup if desired.
For the mango lassi, combine mango, yogurt, milk, cardamom, and sugar in a blender. Blend until smooth and creamy. Taste and adjust sweetness as needed.
Serve the chopped cheese sandwiches with carrots and hummus and mango lassi.