Combine thyme, mustard, cumin, salt, pepper and oil. Rub over the leg of lamb.
Line a baking sheet with a piece of kraft paper large enough to enclose the lamb and onions. Spread the onions in a layer in the center of the paper and lay the lamb on top. Carefully pour the water around the leg.
Fold the paper tightly around the lamb and onions, enclosing them like a package. Make sure the paper is completely sealed. A clean paper grocery bag tied closed is another option.
Place the lamb on the baking sheet in the oven and roast for 8 hours. The lamb is done when the meat pulls away and is tender. Serve the meat from the paper cut into bite sized pieces alongside the caramelized onions or on a platter with baguette chunks, extra mustard and kani sauce on the side.
Tamarind Kani Mustard Sauce
In a saucepan, heat the oil over medium-low heat. Cook the onion and garlic, stirring frequently, until soft and fragrant. Add the tomatoes, Scotch bonnet, and bay leaf. Simmer for 15 minutes, or until thick like a chutney, stirring occasionally with a wooden spoon and adding a little water if necessary.
Add the tamarind, sugar and fish sauce and season with salt and pepper to taste. Remove the bay leaf and puree in a food processor or blender until smooth. Store in the refrigerator in an airtight container for up to 3 weeks.Makes about 1 cup.
Notes
Meat is often cooked in parchment or paper in other parts of the world. Venison or goat meat can be substituted for the lamb.