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1
Bring the 3 quarts of water to a boil in a large pot and add the penne.
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2
Cook for approximately 8 minutes, until just tender, and drain.
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3
Rinse briefly under cool water to stop the cooking if you don't intend to assemble the dish immediately.
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4
Bring 1/3 cup of water to a boil in a large skillet.
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5
Add the broccoli pieces, cover, return to the boil and cook for 3 minutes, or until most of the water has evaporated.
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6
Add the mushrooms, red pepper, walnuts and olive oil and mix well.
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7
Add the spinach, still wet from washing, cover and cook 2 to 3 minutes, or until it is wilted.
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8
(At this point, the vegetables can be combined with the warm pasta, seasoned with salt and pepper and served with a little grated Parmesan cheese as a side dish.)
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9
For the gratin, set the vegetables aside while you make the sauce.
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10
For the bechamel sauce, melt the butter in a saucepan.
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11
Add the flour, mixing it in well with a whisk.
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12
Stir in the milk and bring the mixture to a boil, stirring with a whisk (especially around the edges where it might have a tendency to burn) until lightly thickened.
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13
Add the cream and salt and pepper, then return to the boil.
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14
Set aside.
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15
When ready to serve, mix together the vegetables, pasta and bechamel sauce and stir in the Swiss cheese.
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16
Spread the mixture into a 10-to-12-cup gratin dish.
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17
If the ingredients are still hot, place the dish under a hot broiler for 5 minutes, until the top is lightly browned.
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18
If some of the ingredients are lukewarm or cool, heat the gratin for about 10 minutes in a 400-degree oven until bubbling hot, then brown for 5 minutes under the broiler.
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19
If the mixture is cool throughout, place the gratin in a 400-degree oven for approximately 15 to 20 minutes, then place under the broiler for 5 minutes.
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20
Serve immediately.
