In a medium saucepan bring water to a boil. Add rice and stir. Reduce heat, cover and simmer for 20 minutes.
Preheat oven to 350 degrees F (175 degrees C).
In a large skillet over medium high heat, place the ground lamb, onion, garlic, thyme, basil and ground mustard. Cook until the lamb is evenly brown and the onion is soft. Stir the rice into the mixture.
Stuff the acorn squash halves with the ground lamb mixture. Season with salt and pepper.
Place stuffed squash on a medium baking sheet. Bake in the preheated oven 30 minutes, or until the squash is tender.