Summer Squash Gratin
6 Tbsp. unsalted butter 6 Tbsp. flour 3 cups whole milk 1 bay leaf 1 Tbsp. minced garlic Salt & pepper to taste 2 tsp. chopped fresh thyme 3 Tbsp. chopped fresh flat leaf parsley 1 Tbsp. olive oil 1 onion, thinly sliced 5 zucchini, about 1 1⁄2 lb. total, peeled and sliced into 1⁄4 inch slices 6 yellow squash, about 1 1⁄2 lb. total, peeled and sliced into 1⁄4 inch slices 2 russet potatoes, peeled and sliced into 1⁄4 inch slices 1 cup finely grated Pecorino-Romano cheese
Preheat oven to 350 degrees. Butter a 9x13 inch baking dish.
In a saucepan over medium heat, melt the 6 tablespoons butter. Stir in the flour to make a roux and cook, stirring occasionally, for 2 minutes. Whisk in the milk, bay leaf and garlic until no lumps remain. Bring to a simmer and cook, stirring occasionally, until thickened, about 3 minutes. Remove the bay leaf and discard. Season the sauce with salt and pepper. Stir the thyme and parsley. Remove from heat.
In a sauté pan over medium-high heat, warm the olive oil. Add the onion and cook until lightly browned
and tender, about 7 minutes. Remove from heat.
To assemble the gratin, layer one-third of the zucchini and yellow squash in the bottom of the prepared baking dish. Top with half of the onion, half of the potatoes and 1/3 cup of the cheese. Spread 1 cup of the sauce on top. Repeat the layering with one-third of the squash, the remaining onion and potatoes and 1/3 cup of the cheese. Spread 1 cup of the sauce on top. Layer the remaining squash on top, spreading the remaining sauce over the squash and sprinkle with the remaining cheese.
Transfer the baking dish to the oven. Bake until the squash and potatoes are tender and the top is golden brown, about 1 hour.