Gyros with homemade tzatziki sauce

Ingredients
1 onion medium, roughly chopped
1 lb ground beef
1 lb ground lamb
2 cloves garlic minced 
1 Tbsp marjoram (dried)
1 Tbsp rosemary (dried, or a small sprig of fresh, taken off the main stem)
2 tsp salt sea salt
½ tsp black pepper
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Instructions
Place the onion in a food processor for about 20 seconds. Place the onion in the center of a kitchen or paper towel and squeeze out the juice and return the onion pulp to the food processor. Discard the juice.
Add the meat, garlic, spices, salt and pepper to the onion, and process. Do so until it looks like a fine paste, about a minute. Place the mixture in a loaf pan and press evenly.
Bake in a water bath (place loaf pan inside another pan filled with water) at 350°F for about an hour and 15 minutes, or until the internal temperature reaches 170°F (77°C) when checked with a meat thermometer.
Remove from the oven, drain the liquid, and place the loaf pan in a cool place and cover the meat with aluminum foil for about 15 minutes.
Place meat on a cutting board and slice as thinly as possible with a sharp knife (using anything else would be much more difficult.)
I added the following step myself as I think it makes it a bit more better. Add a few slices to a nonstick frying pan (very little or no oil) and heat gently until the meat browns a little.

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To Serve: Ingredients tzatziki sauce
1 cucumber grated
1 tsp sea salt, or more as needed
16 oz Greek yogurt
2 tsp garlic finely grated
1 Tbsp extra virgin olive oil
leaves lettuce butter or romaine
slices tomatoes
slices sweet onion

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Instructions
Place the shredded cucumber in a colander and sprinkle with sea salt. Place a plate and weight on top to press out the liquid, for about an hour.
Drain, then squeeze the cucumber in a kitchen towel or strong paper towel to remove the rest of the liquid.
Next, place all the ingredients in a mixing bowl, vinegar. Mix well, and refrigerate until ready to use. This sauce is best chilled for before serving.