For the mashed potatoes:
2 large Russet potatoes, peeled and chopped into 1-inch cubes
1/2 cup warm milk
6 tablespoons butter, divided
1/2 cup sour cream
2 to 3 tablespoons chives, chopped
Salt and pepper
2 pounds ground turkey
Salt and pepper
2 rounded tablespoons Dijon mustard
Olive or vegetable oil, for drizzling
3 tablespoons grated white onion
2 rounded tablespoons flour
1 cup beef stock or consommé
1 cup chicken stock
1 tablespoon Worcestershire
6 potato rolls, lightly toasted
Bread-and-butter or dill pickle slices, chopped white onions, thinly sliced tomato and lettuce leaves, for serving
Fill a large pot with water. Add potatoes, salt water and bring to a boil. Boil until potatoes are tender when pierced with a fork, about 10-12 minutes. Drain the potatoes and return them to the hot pot to let them dry out a bit. Mash (for chunkier potatoes) or press through a food mill (for creamier potatoes). Add milk, half the butter, sour cream and chives, and season with salt and pepper. Stir to combine. Cover to keep warm.
Combine turkey with salt, pepper and Dijon then form 6 patties that are thinner at the center and thicker at the edges for even cooking.
Heat a large skillet over medium heat and melt remaining 2 tablespoons butter. When it foams, add grated onion, season with salt and pepper, and stir 2-3 minutes. Add flour and stir a minute more; whisk in beef and chicken stocks and cook until thick enough to coat a spoon, 3-4 minutes. Add Worcestershire sauce to taste.
Heat a cast-iron skillet or griddle over medium-high heat.
Arrange roll bottoms on plates or platters and dress with pickles, onions, tomato and lettuce.
Cook burgers in skillet or on the griddle for 9-10 minutes, turning occasionally.
Place turkey burgers on top of lettuce then top turkey burgers with mashed potatoes and some gravy. Set bun tops in place and serve with grilled corn or sautéed frozen grilled corn kernels dressed with butter, hot sauce and scallions, and steamed broccoli or green beans alongside.