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1
Bring 1 1/4 es water to a simmer in a large pot.
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2
Place potatoes in a steamer basket, cover and steam over medium or medium-low heat, replenishing the water as necessary, until the potatoes are fall-apart tender, 20 to 30 minutes(or you can use steamer).
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3
At the same time, put mushrooms in a food processor and pulse, working in two batches and stopping to scrape down the sides as needed, until the mushrooms are coarsely chopped(or you can use knife to chop).
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4
Then transfer the potatoes to a large bowl.
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5
Add butter and use a folk to mash until chunky smooth.
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6
Slowly stir in buttermilk, egg and egg white and 1 teaspoon salt.
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7
Mix well.
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8
Set aside.
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9
Heat oil in a large skillet over medium heat.
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10
Add shallots, garlic and cook, stirring, until fragrant and starting to be soft, about 1 minute.
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11
Then add the mushrooms and cook, stirring often, until mushrooms release their liquid and the pan is almost dry, about 12 minutes.
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12
Add Swiss chard and go on cooking, stirring, until wilted, about 4 minutes.
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13
Whisk broth and flour in a small bowl.
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14
Add to the pan along with the remaining 1/4 teaspoon salt, black pepper to taste and rosemary.
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15
Cook, stirring, until the mixture bubbles and thickens, about 1 1/2 minutes.
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16
Preheat oven to 400F.
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17
To assemble, spread half of the mashed potatoes in an even layer in a 9-by-13-inch (or 3-quart) baking dish.
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18
Sprinkle half the Parmesan over the potatoes.
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19
Spread the mushroom mixture on top and spread the remaining potatoes over the mushroom layer.
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20
Top with the remaining Parmesan.
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21
Bake until hot throughout and the top is golden brown, about 35 minutes or more.
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22
Cool lightly and serve warm.
