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1
Preheat the oven to 400 degrees F. Bring a large pot of water to boil.
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2
Once boiling, cook the pasta according to package directions until just shy of al dente.
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3
Drain well and set aside.
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4
Meanwhile, cook the bacon in a large skillet or saute pan over medium heat until crispy.
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5
Transfer to a paper-towel-lined plate with a slotted spoon.
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6
(If desired, set aside 2 or 3 tablespoons of the cooked bacon for topping the baked mac and cheese.)
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7
Reserve 2 tablespoons of the bacon grease.
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8
Return the pan to the heat and add the mushrooms.
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9
Cook, stirring occasionally, until tender and most of the liquid has evaporated, about 5-7 minutes.
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10
Remove from the heat.
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11
To make the cheese sauce, add the reserved bacon grease to a medium saucepan with the butter.
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12
(If there was not enough bacon grease to make 2 tablespoons, replace the missing amount by adding an equal amount of butter.)
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13
Heat over medium-high heat until the butter is completely melted.
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14
Whisk in the flour and garlic and cook, stirring constantly, until the mixture is golden and fragrant but not burned, 1-2 minutes.
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15
Whisk in the milk.
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16
Cook the mixture, stirring frequently, until it begins to bubble and thicken.
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17
Whisk in the salt, red pepper flakes, pepper, and cayenne pepper.
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18
Remove the pan from the heat and whisk in the Fontina and white cheddar until completely melted and smooth.
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19
(If necessary, you can return the pan to low heat to melt the cheeses.)
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20
Return the drained pasta to the pot.
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21
Mix in the cooked bacon and mushrooms, the cheese sauce and the chopped spinach.
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22
Stir everything together gently until well combined.
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23
Spread the mixture into a lightly greased 2-quart casserole dish.
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24
Bake for 15-20 minutes or until the top is light golden and the cheese is bubbling.
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25
Serve warm and top with reserved cooked bacon pieces, if desired.
