For the Banana Raita
Combine bananas, yogurt, and cardamom in a medium bowl. Refrigerate while you make the curry.
For the Curry
Season the lamb with salt and pepper. In a large Dutch oven or nonreactive flameproof casserole, heat the oil over moderately high heat. Working in batches, add a single layer of lamb cubes without crowding and brown on all sides (10 minutes). Transfer to a plate. Repeat with remaining lamb cubes.
Add the onions and cook over moderately high heat until golden brown (5 minutes). Add the garlic, serrano, and ginger and cook until fragrant, about 2 minutes. Add the garam masala, coriander, turmeric, cumin, and cloves and cook, stirring constantly, for 1 minute.
Return the lamb to the casserole and stir, coating the meat with the spice mixture and scraping the bottom of the pan to loosen any browned bits. Add the tomatoes, chicken stock, and water and bring to a boil over high heat. Lower the heat to moderate, cover partially and simmer for 1 hour.
Drain the potatoes, pat dry and add to the casserole. Cook until the sauce is thickened slightly, about 10 minutes. Stir in the brown sugar and spinach. Cook until the spinach is wilted and the potatoes are tender (3 minutes). Season with salt and pepper. Serve the curry hot; over rice; pass the banana raita separately.