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1
Preheat an oven to 350 degrees F.
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2
Bring a large saucepan of well salted water to a boil over high heat.
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3
Cut the heads from the broccoli stalks and cut the heads into small florets.
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4
Peel the stalks and thinly slice.
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5
Add the broccoli to the boiling water and cook for 2 minutes.
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6
Transfer the broccoli to a bowl filled with ice water, then drain well.
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7
In a 10-inch fry pan over medium heat, melt the butter.
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8
Add the flour and stir until well incorporated and fragrant, 1 - 2 minutes.
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9
Slowly whisk in the milk and bring to a boil.
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10
Reduce the heat to low, add the onion and bay leaf and simmer, stirring frequently, until the mixture thickens, about 8 minutes.
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11
Remove and discard the bay leaf and add the Gruyere, stirring until the cheese is melted.
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12
Season with salt and pepper.
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13
Carefully fold in the broccoli and spread in an even layer.
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14
In a small bowl, stir together the panko, parmigiano-reggiano and olive oil, and season with salt and pepper.
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15
Sprinkle the panko mixture over the broccoli mixture.
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16
Transfer the pan to the oven and bake until the gratin is golden and bubbly, 40 - 45 minutes.
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17
Let rest for 15 minutes before serving.
