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1
Place a roasting pan in oven and preheat to 425 degrees.
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2
Place cauliflower in a large bowl.
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3
In a separate bowl, mix 3 tablespoons butter, the olive oil, pepper and mustard.
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4
Drizzle about two-thirds of mixture over cauliflower; mix well.
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5
Sprinkle with 1 1/2 teaspoons salt.
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6
Spread cauliflower evenly in heated roasting pan and bake for 10 minutes.
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7
Meanwhile, halve leeks almost to root end, leaving root end in one piece.
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8
In a large bowl, combine leeks, parsnips, remaining 1 teaspoon salt and remaining butter-oil mixture.
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9
Mix well.
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10
Turn cauliflower over in pan, then add parsnips and leeks.
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11
Roast for 20 minutes, turning vegetables halfway through.
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12
Reduce heat to 325 degrees and roast until golden brown and tender, 20 to 30 minutes.
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13
Transfer vegetables to a cutting board and chop into 1/4- to 1/2-inch pieces.
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14
Set aside.
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15
In a medium bowl, combine egg yolks and cream or milk and set aside.
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16
In a small saucepan, bring broth to a simmer.
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17
In another small saucepan over medium-low heat, whisk remaining 3 tablespoons butter with flour.
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18
Continue whisking for about 3 minutes; mixture should bubble gently but not brown.
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19
Remove from heat and slowly add broth, whisking to incorporate.
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20
Continue until all broth is used and sauce is smooth and creamy.
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21
Return to low heat and cook until slightly thickened, 2 to 3 minutes.
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22
Whisk into bowl of egg yolks and cream.
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23
Return to pan over medium-low heat just until steaming, 2 to 3 minutes; do not boil.
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24
Reserve 1 cup sauce for garnishing filled crepes.
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25
Add chopped vegetables to remaining sauce.
