In a blender, combine frozen mango, plain yogurt, milk, ground cardamom, and sugar.
Blend until smooth. Adjust sweetness and consistency with more sugar, milk, mango, or a splash of water if needed.
In a bowl, combine all Chashu beef ingredients. Submerge the sirloin steak in the liquid and cover with a lid.
To make the broth, preheat a pot with vegetable oil over medium-low heat. Sauté the onion, garlic, and ginger for 5 minutes, stirring often.
Add mushrooms, leek, white parts of scallions, and broth to the pot, ensuring vegetables are covered by broth.
Bring to a rolling boil over high heat, then reduce to a mild simmer and cover with a lid. Add the marinated beef into the pot.
Cook until the broth reduces by about half.
Place eggs into the broth and simmer for 6 minutes. Transfer to an ice bath, cool, then peel. Add ramen noodles to the simmering broth and cook according to package instructions.
Portion broth into bowls. Add noodles and top with Chashu beef, eggs cut in half, and scallions for garnish. Serve with mango lassi.