Golden Broccoli Gratin

INGREDIENTS

• Kosher salt, to taste
• 2 lb. broccoli
• 2 Tbs. unsalted butter
• 1/4 cup all-purpose flour
• 2 cups milk
• 1/2 small yellow onion, thinly sliced
• 1 bay leaf
• 1 cup grated Gruyère cheese
• Freshly ground pepper, to taste
• 1 cup panko
• 1/4 cup grated Parmigiano-Reggiano cheese
• 2 Tbs. olive oil