Heat the olive oil in a large (12-inch) saute pan over medium heat. Add the onion and celery and cook for 5 to 7 minutes, stirring occasionally, until the onion is translucent but not browned. Set aside to cool slightly.
Place the beef, veal, pork, parsley, thyme, chives, eggs, milk, salt and pepper in a large mixing bowl. Put the panko in a food processor fitted with the steel blade and process until the panko is finely ground.
Add the onion mixture and the panko to the meat mixture. With clean hands, gently toss the mixture together, making sure it's combined but not compacted.
Place a piece of parchment paper on a sheet pan. Pat the meat into a flat rectangle and then press the sides in until it forms a cylinder down the middle of the pan (this will ensure no air pockets). Bake for 40 to 50 minutes, until a thermometer inserted in the middle reads 155 degrees F to 160 degrees F. Remove from the oven and allow to rest for 10 minutes. Slice and serve hot with the Garlic Sauce.
Combine the oil and garlic in a small saucepan and bring to a boil. Lower the heat and simmer for 10 to 15 minutes, until lightly browned. Be careful not to burn the garlic or it will be bitter. Remove the garlic from the oil and set aside. (I save the oil for vinaigrettes.)
Combine the chicken stock, butter and cooked garlic in a medium saucepan and bring to a boil. Lower the heat and cook at a full boil for 35 to 40 minutes, until slightly thickened. Mash the garlic with a fork, whisk in 1/2 teaspoon salt and 1/4 teaspoon pepper, and taste for seasonings. Spoon the warm sauce over the meatloaf.