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1
Melt butter in a large, heavy skillet over medium heat.
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2
When hot, add the onions, stir well to coat, cover with a lid and allow to cook for 15 minutes without stirring.
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3
In the meantime, bring the stock, 3 cups of milk and the salt to a boil in a medium pot over high heat.
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4
Add the polenta, stir well, bring back to a boil, then lower heat and simmer for 20-30 minutes, stirring occasionally.
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5
Flip the onions so the browned ones on the bottom are now on the top.
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6
Re-cover and cook for 15 more minutes without stirring.
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7
When the polenta is creamy and thickened, remove from heat and pour onto an oiled sheet pan.
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8
Spread evenly over the pan, season with coarse salt and allow to cool and set completely about 45 minutes to an hour.
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9
While the polenta is cooling, mix together the mascarpone, crumbled blue cheese and 1/4 cup milk until combined but still chunky.
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10
Add a little more milk if it's too thick to spread easily.
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11
Season with salt and pepper, cover and set aside.
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12
Preheat the oven to 375F.
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13
Trim the cooked polenta to make two rectangular slabs you can layer in your baking dish, sliding a spatula under the slabs to loosen them from the pan without breaking them.
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14
If they do crack or separate, it's not a big deal you can just reassemble them in the baking dish.
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15
Spread the bottom of the baking dish with half the pesto, then transfer the first polenta layer on top.
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16
Spread an even layer of the blue-cheese mixture on top of the polenta, then add a thin layer of caramelized onions and sprinkle lightly with Parmesan.
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17
Top with prosciutto, spread the rest of the pesto on top, then add the second layer of polenta.
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18
Spread the top of the casserole with the remainder of the blue-cheese mixture and top with the rest of the Parmesan.
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19
Bake for 20 minutes, then remove, preheat the broiler and broil on high heat for 5 minutes or until the top is browned and bubbling.
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20
Remove and allow to cool and set for 10 minutes before slicing and serving.
