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1
Place the potatoes in a saucepan, and cover with water.
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2
Add 1/2 teaspoon salt, and bring to a boil.
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3
Reduce the heat to medium, and boil gently until tender, 10 to 20 minutes, depending on the size of the potatoes.
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4
Add the chard stalks to the pot, and simmer for another five minutes until crisp-tender.
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5
Drain, and retain the cooking liquid if you prefer to use it instead of milk for the bechamel.
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6
Cut the potatoes into quarters or halves, depending on their size.
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7
Preheat the oven to 425 degrees.
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8
Rub a 2-quart baking dish or gratin with the cut clove of garlic, and brush with olive oil.
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9
Make the bechamel.
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10
Heat the oil over medium heat in a heavy medium saucepan.
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11
Add the shallot or onion, and cook, stirring, until softened, about three minutes.
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12
Stir in the flour, and cook, stirring, for about three minutes until smooth and bubbling but not browned.
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13
It should have the texture of wet sand.
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14
Whisk in the milk all at once, and bring to a simmer, whisking all the while, until the mixture begins to thicken.
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15
Turn the heat to very low and simmer, stirring often with a whisk and scraping the bottom and edges of the pan with a rubber spatula, for 10 to 15 minutes until the sauce is thick and has lost its raw flour taste.
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16
Season with salt and pepper.
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17
Strain while hot into a heatproof bowl or a Pyrex measuring cup.
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18
Cut the potatoes into quarters or halves, depending on their size.
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19
Combine with the chard stalks and thyme in a large bowl, and season with salt and pepper.
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20
Add the bechamel, and stir until the vegetables are coated with sauce.
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21
Scrape into the gratin dish.
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22
Sprinkle the Gruyere on top.
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23
Bake 20 to 25 minutes until bubbling and beginning to brown on the top.
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24
Serve hot.
