1 ½ lb. baby white or yellow potatoes
4 cloves garlic, crushed
¼ cup chopped fresh chives
3 tbsp. butter
4 tbsp. butter, cut into pieces
1 large or 2 medium onions, chopped
¼ lb. white mushroom caps, chopped or sliced
1 tbsp. fresh thyme or 1 tsp. dried thyme
1 tbsp. thinly sliced fresh sage or 1 tsp. dried sage
1 bay leaf
Salt and pepper
3 tbsp. flour
2 cups beef stock
About 1 tbsp. soy sauce or shoyu (Japanese soy sauce)
1 ½ lb. ground sirloin
Kosher salt and coarsely ground black pepper
½ cup dried breadcrumbs
1 large egg, beaten
About 2 tbsp. Worcestershire sauce
2 tbsp. tomato paste, whisked with 3 tbsp. hot water
1 tbsp. (a scant palmful) dry mustard
1 tsp. (1/3 palmful) granulated garlic
1 tsp. (1/3 palmful) granulated onion
2 tbsp. olive oil
Finely chopped parsley, for serving
In a large saucepan, cover the potatoes and garlic with water and bring to a boil. Salt the water and cook, uncovered, at a rolling boil until tender, about 12 minutes. Drain the potatoes and garlic and return them to the hot pot. Using a potato masher, coarsely crush the potatoes and garlic. Mix in the chives and butter; season with salt and pepper. Cover to keep warm.
While the potatoes come to a boil, make the gravy. In a large skillet, melt the butter over medium heat. Add the onion, mushrooms, thyme, sage, and bay leaf. Cook, stirring often, until the onion and mushrooms are light caramel brown in color, 15 to 18 minutes; season with salt and pepper. Sprinkle with the flour. Cook, whisking often, until the flour is absorbed, about 1 minute. Whisk in the stock. Cook, whisking often, until thickened, 4 to 5 minutes. Season to taste with the soy sauce.
While the gravy cooks, heat a large cast-iron or nonstick skillet over medium-high. Place the beef in a large bowl; season with kosher salt and black pepper. Mix in the breadcrumbs, egg, Worcestershire, tomato paste, dry mustard, and granulated garlic and onion. Form the beef into 4 oval-shaped patties (thinner in the centers for even cooking).
Add the oil, two turns of the pan, to the skillet. Cook the patties for 10 to 12 minutes, turning occasionally.
Serve the potatoes alongside the patties. Top the patties with the gravy and parsley.