Melt the butter in a Dutch oven over medium heat. Once it's melted, add the mushrooms, leek, and garlic. Cook until the mushrooms have released their juices and the liquid has almost evaporated, 10-12 minutes. Sprinkle the flour over the vegetables and cook for 3 minutes more, stirring constantly.
Pour the broth, milk, wine, and cream into the Dutch oven. Increase the heat to medium-high and bring to a boil, then reduce heat to low and simmer for 5 minutes. Remove from heat.
Stir in the Fontina, half of the Parmesan, and the smoked paprika. Add the cooked farfalle and asparagus to the Dutch oven; season to taste with salt and pepper.
Spray a large casserole dish with an oil mister. Pour the tetrazzini mixture into the dish and top with the panko and remaining parmesan cheese. Spray the top with additional oil and place the casserole in the oven uncovered. Bake for about 40 minutes, until the sauce is bubbling and the top is golden brown. Let the casserole cool for 5 minutes, then sprinkle with parsley before serving.
Craving more veg-friendly recipes? Shop our collection of vegetarian and vegan cookbooks, including our new Vegan Thanksgiving and Vegan Christmas cookbooks!