Shepherd's Pie

2 T olive oil
3 T unsalted butter
1 medium onion, diced
2 medium carrots, peeled and sliced
1 rib celery, sliced
1 can mushrooms
2 cloves garlic, minced
3/4 tsp dried oregano
1/2 tsp salt
3 T tomato paste
1 pound ground beef (some could be sausage)
1/2 cup canned beef broth
1 tsp Worcestershire sauce
black pepper to taste
4 cups mashed potatoes & 1/2 tsp salt
1/4 c grated Parmesan cheese, optional

Preheat oven to 375 degrees.

Heat olive oil and 2 T of the butter in a large skillet over medium heat. Add the onion, carrots, celery, mushrooms, garlic, 1/2 tsp salt and oregano. Cook until vegetables are tender, about 10 minutes. Stir in the tomato paste and cook until mushrooms and paste are warmed, about 5 minutes. Stir in the beef, the broth, the Worcestershire, and some pepper, breaking up any large clumps of meat, cook until the meat is no longer pink.

Transfer meat and vegetables to a 2 quart casserole dish and spread the mashed potatoes over the top, leaving a 1/4 inch border around the edge. If desired, sprinkle with Parmesan. Dot with 2 T butter. Bake until potatoes brown and the juices bubble around the edge, about 30-40 minutes.