Summer Squash Gratin
- 6 Tbs. (3/4 stick) unsalted butter, plus more for greasing pan
- 1/3 cup all-purpose flour
- 3 cups half-and-half
- 1 bay leaf
- 1 Tbs. minced garlic
- Kosher salt and freshly ground pepper, to taste
- 2 tsp. chopped fresh thyme
- 3 Tbs. chopped fresh flat-leaf parsley
- 1 Tbs. olive oil
- 1 yellow onion, thinly sliced
- 5 zucchini, about 1 1/2 lb. total, peeled into wide, thin sheets
- 6 yellow squash, about 1 1/2 lb. total, peeled into wide, thin sheets
- 2 russet potatoes, skin peeled and discarded, potatoes peeled into
wide, thin sheets
- 1 cup finely grated Parmigiano-Reggiano cheese
In a saucepan over medium heat, melt the 6 Tbs. butter. Stir in the flour to make a paste and cook, stirring occasionally, for 2 minutes. Whisk in the half-and-half, 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 in the thyme and parsley. Let the sauce cool to room temperature.
In a sauté pan over medium-high heat, warm the olive oil. Add the onion and cook, stirring occasionally, until lightly browned and tender, about 10 minutes, adjusting the heat as needed to prevent overbrowning. Remove from the heat.
To assemble the gratin, layer one-third of the squash (both 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, spread 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. Serves 10 to 12.