Preheat the oven to 375°F. In a large bowl, combine the ground beef, breadcrumbs, milk, diced onion, 2 minced garlic cloves, 1/4 cup feta, kalamata olives, parsley, oregano, and a pinch of salt and pepper. Mix until evenly blended, then shape into a loaf and place on a baking sheet.
Bake for 45 to 50 minutes until the internal temperature reaches 160°F. While the meatloaf cooks, prepare the tzatziki. Stir together the Greek yogurt, half the grated cucumber, remaining minced garlic clove, lemon juice, dill, and a pinch of salt in a small bowl. Set aside.
For the salad, toss the cherry tomatoes, remaining grated cucumber, red onion slices, olive oil, dried oregano, and a pinch of salt and pepper in another bowl. Once the meatloaf finishes baking, let it rest for 5 minutes before slicing. Serve slices with tzatziki on the side and the salad.