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1
Roast and cool the beets in advance.
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2
Trim the tail and top of the beets.
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3
Season the beets still in their skins (salt, pepper and olive oil) and roast until tender (about 1 hour or so).
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4
Cool, peel while warm, slipping skins off under running water and slice about 1/2 inch thick (1 cm or so).
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5
Preheat oven to 375F.
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6
Coat four 10-ounce ramekins (or a 2 to 2 1/2-quart souffle dish) with cooking spray and place them on a baking sheet.
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7
Line the bottom with the sliced beets.
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8
Set aside.
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9
Melt butter and oil in a large saucepan over medium-high heat.
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10
Whisk in flour and cook, whisking, for 1 minute.
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11
Adjust heat as needed to prevent the mixture from getting too dark; it should be the color of caramel.
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12
Add milk, mustard, sage and salt and cook, whisking constantly, until thickened, 1 to 2 minutes.
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13
Remove from heat and immediately whisk in goat cheese and 3 egg yolks until well combined.
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14
Transfer to a large bowl.
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15
Beat the 5 egg whites in a medium bowl with an electric mixer on high speed until soft peaks form.
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16
Add cream of tartar and continue beating until stiff peaks form.
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17
Using a rubber spatula, gently fold half of the whipped whites into the milk mixture.
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18
Gently fold in the remaining egg whites just until no white streaks remain.
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19
Transfer to the prepared ramekins or souffle dish.
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20
Bake until puffed, firm to the touch and an instant-read thermometer inserted into the center registers 160F, about 20 minutes in ramekins or 30 minutes in a souffle dish.
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21
Serve immediately.
