Preheat the oven to 350 degrees F.
Rinse the grouse and pat dry. Season with salt and pepper inside and out. Secure the legs of the birds to the breast with toothpicks or kitchen twine.
Stuff the cavity of the birds with equal parts apple, pear, and onion, and lay them in a large roasting pan or baking dish. Add a few garlic cloves to the cavity and scatter the rest in the pan along with the remaining apple, pear, and onion.
Add the orange juice to the bottom of the pan along with the butter.
Cook the birds for 45 minutes, or until a digital meat thermometer reads 165 degrees F when inserted between the leg and the breast meat. While cooking, baste the birds every 7-10 minutes with a large spoon, making sure to distribute the juices over the breast meat in particular.
Remove from the oven and cover with tin foil. Let sit for 15 minutes before carving and serving!