In a large bowl, whisk the eggs. Then add the panko, parsley, yogurt, 2 tablespoons of the olive oil, garlic, shallots, onion powder (if using), thyme, celery seeds, rosemary, salt and several generous grinds of black pepper. Stir well with a fork into a thick, uniform mixture with the panko evenly moistened and the seasonings evenly distributed. Add the turkey and mix well with your hands. Roll the mixture into 10 meatballs, each one about 2½ inches in diameter, and put them on a plate.

Warm the remaining 2 tablespoons oil in a 12-inch skillet over medium-high. Arrange the meatballs in the skillet (it will be snug but they should all fit) and sear, without disturbing, until the meatballs are deeply browned on one side, about 3 minutes.

Flip the meatballs, adjust the heat to medium-low, and cover the pan. Cook until the meatballs are cooked through (an internal temperature of at least 160 degrees) and the second side is lightly browned, 12 to 15 minutes. Remove the meatballs to a serving platter and cover to keep warm.

Melt the butter in the same skillet over medium heat. Add the flour and whisk well to make a uniform paste that smells lightly toasted, about 1 minute. Increase the heat to medium high, add the chicken broth and whisk well to smooth the gravy, scraping up the bits on the bottom of the pan. While whisking, let the gravy come to a bubble and thicken, 1 to 2 minutes. Squeeze in the lemon juice and season to taste with salt and pepper. Serve the meatballs with the gravy.