Prepare Sauce: Mix mayonnaise, ketchup, and mustard in a small bowl. Set aside.
Caramelize Onions: Heat oil in a skillet over medium heat. Add onion and sauté for 5–7 minutes until golden and caramelized. Remove from heat.
Shape and Season Patties: Divide ground beef into 2 patties per portion. Shape into patties slightly larger than buns. Season with salt and black pepper.
Cook Patties: Heat a grill pan or skillet over medium-high heat. Cook patties for 2 - 3 minutes on one side. Flip and cook to desired doneness. Top each patty with a cheese slice.
Toast Buns: Toast bun halves cut-side down until golden.
Assemble Burgers: Spread sauce on the bottom bun. Add a patty, tomato slices, caramelized onions, and optional pickles. Top with the other bun half.
Make Mango Lassi: Blend mango, yogurt, milk, cardamom, and sugar until smooth. Adjust sweetness to taste.
Serve: Plate burgers with carrots, ranch, and mango lassi.