Here's how I have always made them. The seasoning is the way my family likes it but you can adjust to your taste.
GYROS
1 tsp. fennel seeds
2 Tbs. boiling water
1 Tbs chopped (very fresh) garlic
2 onions, finely chopped
½ cup dry whole wheat bread crumbs
2 eggs
¼ tsp. dried thyme
1 tsp. dried sweet marjoram
¼ tsp. ground cinnamon
1 tsp. cumin powder
1/8 tsp. cayenne pepper
2 ½ lbs. ground lamb, at least 1/6 fat by weight
½ lb. ground beef
½ cup minced parsley
¼ lb. fresh pork fat
pita bread
Slightly crush the fennel seeds and soak in the boiling water for 15 minutes.
Process the garlic and onions together in a food processor or blender. Add the bread crumbs and eggs and mix until just combined. Add the fennel seeds and water, thyme, marjoram, cinnamon, cumin and cayenne and blend for 1 minute.
Combine the garlic mixture with the lamb, beef and parsley. Mix until thoroughly combined but do not overmix. Fry a small amount to check the seasonings and add more if necessary.
Preheat the oven to 350 degrees. Thinly slice the pork fat and place a single layer on the bottom of a 9 X 5 inch loaf pan. Spoon in the meat mixture and top with another layer of pork fat. Cover the pan with foil. Crimp it near the top, or water could seep into the loaf.
Place the loaf pan in a roasting pan. Pour boiling water into the roasting pan to halfway up the sides of the loaf pan. Do not let the loaf pan float. You might need to put a custard cup weighted with dry beans on top if it floats. Bake for 1 ½ to 1 ¾ hours, until cooked through but not dry.
Remove the pork slices on top. Do not drain the fat that forms around the loaf, as it helps to preserve the meat. Replace the foil and set another loaf pan, right side up, on top of the loaf, Weight it by putting several cans, or a large peanut butter jar filled with water in the empty loaf pan. Weight the loaf for 2 to 3 hours, then remove the weights.
Refrigerate, then unmold when chilled. Wrap in plastic wrap, then foil, and store in the refrigerator.
To serve, thinly slice the meat and grill or broil until the edges are slightly crispy. Some of the fat will melt out. Stuff into pita bread and serve with the usual accompaniments, like garlic yogurt sauce.
Gyro loaf will keep in refrigerator for up to 1 week if tightly wrapped or frozen up to 3 months.