1 yellow onion, chopped
2 leeks, chopped, optional
1 potato, diced or sliced
3/4 cup baby portobello mushrooms, optional
1 teaspoon salt
1/2 teaspoon pepper
1/2 teaspoon garlic powder
2-3 tablespoons extra-virgin olive oil
Sauce:
1/4 cup (1/2 stick) unsalted butter
1/4 cup all-purpose flour
1/2 cup heavy cream
1/2 cup whole milk
1 1/4 cups sharp cheddar cheese, grated, divided
3/4 cup gruyere cheese, grated
1/2 teaspoon salt
1/4 teaspoon pepper
1/4 teaspoon nutmeg
Preheat oven to 350º F and lightly grease baking dish (9x13-inch, or other, depending on what you have) with butter or non-stick spray. Bring a large pot of salted water to boil and cook pasta 1-2 minutes less than package instructions. Drain, rinse with cool water and set aside. Heat olive oil in a large pan or skillet over medium-high heat and sauté potato for 10 minutes, then add onion, leek and mushrooms until softened. Potatoes should be fork tender. Season with salt, pepper and garlic powder, then remove from heat and set aside. In a separate, large saucepan, heat butter over medium heat until melted. Whisk in flour and cook, stirring frequently, until lightly browned and smooth, 2-3 minutes. Season with salt, pepper, and nutmeg, then slowly whisk in heavy cream and whole milk. Continue to cook until mixture is smooth and slightly thickened, then stir in 3/4 cup cheddar cheese and all of gruyere, whisking until melted. Stir cooked pasta and sautéed veggies into cheese sauce, until everything is thoroughly coated, then transfer to greased baking dish. Top with remaining cheddar cheese, then place in oven. Bake for 35-40 minutes or until top is melted and bubbly and dish is warmed throughout. Remove from oven and serve hot.