Mushroom and Spinach Tart with Walnuts

This ridiculously delicious tart owes its creation to the popularity of the previous night's enchiladas (butternut squash and black beans: watch this space). Which is to say that the leftovers I'd counted on failed to materialize, necessitating a Whole New Dinner. A quick scan of available supplies revealed a plethora of mushrooms, a package of vegan cream cheese, and some frozen puff pastry I'd been meaning to use; the addition of caramelized onions, a bit of spinach, and a handful of walnuts resulted in what I can only describe (albeit immodestly) as a triumph of MacGyver cuisine. My only quibble was that I wished there were twice as much of the stuff; next time I'll remedy that problem by doubling the recipe.

Mushroom and Spinach Tart with Walnuts
Ingredients
~ 1 package (2 sheets) frozen puff pastry, thawed
~ 1.5 lbs. mushrooms, sliced
~ 1 tbsp. olive oil
~ 1 large onion, sliced into thin crescents
~ 4 cloves garlic, minced
~ 1 tsp. each: salt, tarragon
~ 1/2 tsp. white pepper
~ 1/4 cup white wine
~ 1/3 cup chopped, fresh sage
~ 1/2 lb. chopped frozen spinach
~ 1/4 cup each: flour, nutritional yeast
~ 8 oz. vegan cream cheese
~ 1/2 cup chopped walnuts

Directions
~ Preheat the oven to 350 degrees fahrenheit and lightly coat two baking sheets with cooking spray.
~ Coat a large skillet with cooking spray and sauté the sliced mushrooms over medium heat for about 10 minutes, stirring occasionally, until they have released their liquid and browned. Transfer to a plate.
~ In the same skillet, heat the olive oil and sauté the onions over medium heat, stirring occasionally, for 15-20 minutes, until soft and golden.
~ Add the garlic, salt, tarragon, pepper, and cook another minute or so. Pour in the wine to deglaze the pan, making sure to scrape up any bits that have stuck to the bottom.
~ Add the spinach and fresh sage and continue cooking just until the spinach wilts.
~ Stir in the flour, nutritional yeast, and cream cheese, and continue cooking about 5 minutes more, until the mixture thickens. Stir in the cooked mushrooms, remove from heat, and set the mixture aside to cool at least 10 minutes.
~ Arrange the thawed puff pastry sheets on the baking sheets; gently fold in the edges to make a rim and crimp slightly with your fingers.
~ Divide the vegetable mixture equally between the two sheets of puff pastry, smoothing with a spatula to get the filling right to the edges.
~ Bake at 350 degrees for 25 minutes, and then sprinkle the chopped walnuts evenly over the two tarts.
~ Continue baking another 10 minutes, until the crust around the edges is puffed and golden brown and the walnuts are giving off a toasty aroma.
~ Remove from the oven and allow to cool for 10 minutes before slicing.