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1
Preheat oven broiler and move rack about 6 inches from heat.
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2
Wipe the mushrooms clean and pop out the stems, using a paring knife to remove any stem stuck in the cap.
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3
Chop stems small and set aside.
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4
Place caps dome up on a nonstick (or lightly oiled) rimmed baking sheet.
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5
Spray or brush tops with oil and sprinkle with pepper.
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6
Broil about 5 minutes, until caps just begin to soften and release a little liquid.
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7
Set oven to 375 degrees F and set mushrooms aside.
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8
Heat 1 tablespoon butter or vegetable oil in a 10-inch skillet over medium heat.
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9
Add mushroom stems and saute, stirring infrequently, until browned and tender, 710 minutes.
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10
Scoop out of the skillet and into a mixing bowl, leaving extra oil in the skillet.
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11
If no oil remains, add 1/2 tablespoon.
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12
Raise heat to medium high and add frozen beans.
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13
Cook until tender and any water has evaporated, about 510 minutes.
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14
Add to bowl with the mushroom stems.
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15
Carefully wipe out skillet with a damp cloth.
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16
Heat milk until warm, about 2 minutes in the microwave at 50% power.
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17
Melt butter in the skillet over medium low, then add shallots.
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18
Cook a few minutes, until tender but not browned, then sprinkle flour and mustard powder into the pan.
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19
Stir constantly for 23 minutes, until flour is no longer grainy and has darkened slightly.
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20
Reduce heat to low, then slowly whisk in milk.
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21
Raise heat gradually to bring to a simmer while whisking.
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22
Adjust heat to maintain a gentle simmer, stirring frequently, until thickened, about 5 minutes.
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23
Off heat, stir in 13 cup of the Parmesan, 3/4 teaspoon salt, white pepper (a little more if using black), paprika, and parsley.
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24
Adjust salt and pepper to taste.
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25
Turn the mushroom caps face up.
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26
Mix half the sauce with the bean mixture.
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27
Pile into the mushroom caps, packing it into a mound.
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28
Bake for 15 minutes, until mushrooms are juicy and tender.
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29
Mix remaining Parmesan with the fried onions.
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30
Remove tray from oven, sprinkle mushrooms with fried onion mixture, and bake or broil another few minutes, until browned.
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31
Cool slightly before sprinkling with more parsley and serving warm.
