In a bowl mix together onion, the lamb, garlic, Oregano, Parsley, paprika, salt and pepper and knead with hands until you combine all the ingredients together into fine paste.
Preheat the oven to 325 degrees F.
Place the lamb mixture into a loaf pan, making sure to press into the sides of the pan.
Place the loaf pan into a water bath and bake for 60 to 75 minutes or until the mixture reaches 165 to 170 degrees F.
Remove from the oven and drain off any fat. Place the loaf pan on a cooling rack and place a brick wrapped in aluminum foil directly on the surface of the meat and allow to sit for 15 to 20 minutes, until the internal temperature reaches 175 degrees F.
For Tzatziki sauce, combine in a bowl greek yogurt, cucumber, salt, garlic, olive oil and vinegar. Serve as a sauce for gyros. Store in the refrigerator in an airtight container for up to a week.
Slice the meatloaf and serve on tortilla with tzatziki sauce, fresh arugula, tomatoes and pommes frite.