Summer Squash Gratin

INGREDIENTS

• 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