Leek and Mushroom Bake with Polenta Crust

Though there are several assembly steps to this recipe , it can all be made ahead of time and didn't take that long to put together once I got rolling.  I'd never roasted leeks or celery before, and I added some bites of celeriac that were calling from the fridge and it worked nicely.  Everything came together easily and tasted great.  The dried mushrooms have a stronger flavor and add texture that fresh mushrooms wouldn't, though I might try fresh mushrooms next time to tame down the flavor a little. 

ingredients:
3 ribs celery
2 leeks, white part only
1 Tablespoon olive oil
1 Tablespoon butter
2 cloves garlic, peeled
2 cups dried mushrooms 
1 recipe polenta crust
1 recipe Bechamel sauce
1/2 cup grated Parmesan cheese

STEP 1: POLENTA CRUST
1 1/2 cups coarse polenta
6 cups water
1 teaspoon salt
2 tablespoons butter

Bring water and salt to a boil in a heavy bottomed sauce pot. Over medium heat, whisk polenta into boiling water, a handful at a time, until it is all incorporated.

Reduce heat to medium-low and cook polenta, stirring frequently, for about 30 minutes. It will become quite thick. Tip: While your polenta is cooking, prepare steps 2 and 3 of this recipe.

Remove polenta from heat and beat in butter. Pour onto a buttered baking sheet and spread out with a spatula into a rough 9×13 rectangle. Polenta will be moldable.

Cover with plastic wrap and refrigerate until firm or up to two days.

STEP 2: ROASTED VEGETABLES & SAUTEED MUSHROOMS
Preheat oven to 400°F and lightly oil a baking sheet or roasting pan.
Wash and dry celery and leeks. Slice leeks into 1/4 inch ‘coins’. Chop celery ribs into 1/4 inch pieces also.
Place vegetables in a single layer on the baking sheet and roast in the oven for 15 minutes. Give the pan a shake and rotate it at least once during the cooking process.
Soak mushrooms in warm water for 20 minutes (Tip: you can make your Bechamel sauce during this time).
Melt 1 tablespoon butter in a medium sauce pan. Bang garlic cloves with the flat side of a knife to bruise them and toss them into the butter.
Drain mushrooms, squeezing out as much liquid as possible, and add them to the garlic butter.
Season mushrooms with salt and pepper and cook over medium heat for about 7 minutes.

STEP 3: BASIC BECHAMEL SAUCE
3 Tablespoons butter
1/3 cup all-purpose flour
2 1/4 cups milk
1/2 teaspoon salt

Melt butter in a medium pan or sauce pot over medium heat. Dump the flour in all at once and whisk to combine. Cook, stirring constantly, for about two minutes to develop a little more flavor and avoid a ‘floury’ taste in your bechamel.
Gradually add in the milk, whisking vigorously to emulsify sauce and eliminate lumps. Once all the milk has been added, bring to a slow boil and stir until thick and smooth.
Season with salt to taste and reserve until ready to use.

STEP 4: ASSEMBLY AND BAKING
Preheat over to 350°F and butter a 9×13 baking dish.
Combine roasted celery and leeks in the dish and spread out to cover the bottom in a single layer. Top with the sauteed mushrooms, uniformly scattered throughout.
Pour bechamel sauce over the vegetables and distribute evenly using a rubber spatula. Sprinkle with 1/4 cup grated Parmesan cheese.
Cut chilled polenta into rectangles and arrange on top of the bechamel. Sprinkle with remaining 1/4 cup of Parmesan cheese.
Place dish in oven and bake, uncovered, for about 40 minutes, until top is browned and the bechamel is bubbling up the sides.
Serve at once.