Rach on the Radio - Sheperd's Pie Burgers

Shepherd’s Pie Knife-and-Fork Burgers

• 2 tablespoons olive or vegetable oil, plus some for drizzling
• 1/4 pound button mushrooms, 5 to 6, stemmed and very finely chopped
• 1 carrot, very finely chopped
• 1 rib celery, very finely chopped
• 1 shallot, very finely chopped
• 2 to 3 cloves garlic, very finely chopped
• Salt and pepper
• 2 large or 3 medium russet potatoes, peeled and cubed
• 1 1/2 pounds coarse beef chuck
• 4 tablespoons Worcestershire sauce, divided
• 1/4 to 1/3 cup whole milk
• 2 tablespoons sour cream
• 2 tablespoons horseradish
• 1 egg yolk, beaten
• 4 tablespoons butter, divided
• 2 tablespoons finely chopped chives
• 2 tablespoons flour
• 1 can beef consommé
• 1 tablespoon Dijon mustard
• Chopped flat-leaf parsley, garnish
• 4 slices white bread, toasted and lightly buttered