(I discovered this recipe a week or so ago, and made it with fabulous results. Very easy and perfect on a cold night.)
2 tablespoons extra-virgin olive oil
3 tablespoons unsalted butter
1 medium onion, diced
3 medium carrots, peeled and sliced
1 rib celery, sliced
1/2 pound cremini mushrooms, quartered
2 cloves garlic, minced
1 tablespoon kosher salt
3/4 teaspoon dried oregano
3 tablespoons tomato paste
1 pound ground beef
1/4 cup canned low sodium beef broth
1/2 teaspoon Worcestershire sauce
Freshly ground black pepper
4 cups Mashed Potatoes
1/4 cup grated Parmesan, optional
Preheat oven to 375 degrees F.
Heat the olive oil and 2 tablespoons of the butter in a large skillet over medium heat. Add the onion, carrots, celery, mushrooms, garlic, half the salt, and oregano. Cook until vegetables are tender, about 10 minutes. Stir in the tomato paste and cook until mushrooms are soft and tomato paste has turned brick red, about 8 minutes more. Stir in the beef, the broth, the remaining salt, the Worcestershire, and some pepper, breaking up any large clumps of meat, cook until the meat is no longer pink, about 3 minutes.
Transfer the meat and vegetables to a 2-quart oval casserole dish and spread the mashed potatoes over the top, leaving a 1/4-inch boarder around the edge. Make a decorative pattern on the top of the potatoes, if desired. Sprinkle with cheese, if using, and dot with the remaining tablespoon of butter. Bake until potatoes brown and the juices bubble around the edge, about 40 minutes. Let cool for 10 minutes before serving.