Heat a small skillet over medium heat and add the onions and olive oil. Season with salt and pepper and cook until soft and starting to brown. Remove from the heat and reserve. Place the breadcrumbs in a bowl and pour the milk over top. Soak for 10 minutes and squeeze out the milk. Combine the onions and breadcrumbs with the ground duck, pork, egg, parsley, thyme and garlic. Season with salt and pepper and mix with your hands to evenly distribute the ingredients. Heat up a small skillet and cook a small piece of the meatball mixture. Taste and adjust seasoning with salt and pepper.
To assemble the meatballs, portion mixture into 36-40 scoops. Dice the foie gras into ½” pieces. Rub the palm of your hands with a little oil to keep the mixture from sticking. Place a scoop of the meatball mixture in the palm of you hand. Set a piece of foie gras in the center and roll between both your palms to fully enclose the foie gras. Repeat with the remaining meatballs. Reserve.