Weeknight Shepherd’s Pie with Leftover Roast Beef

Ingredients

• 3 medium potatoes, peeled
• 1/2 cup milk
• 2 tablespoons butter
• 1/2 teaspoon salt
• 1/4 teaspoon white pepper
• 1 tablespoon vegetable oil
• 1/4 cup onion, chopped ((fresh or frozen))
• 4 ounces fresh mushrooms, sliced or chopped (or 2 oz. canned mushrooms, sliced)
• 1 cup frozen peas and carrots
• 1 .87 oz package brown gravy mix + 1 cup water ( (or use leftover gravy))
• 1 package beef roast au jus-17 ounces ((or 2-3 cups leftover beef roast, fork-shredded or cut into chunks))
• 1/4 teaspoon dried thyme
• freshly-ground pepper
• paprika
• 2 tablespoons butter (thin slices)