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Start the pasta water heating up. Be sure to add salt.
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Place the jalapeño, lime zest, juice of 1 lime, green onions and garlic into a food processor. Scoop out the flesh of 2 of the avocados and add them as well, and puree until smooth. Set aside.
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Cook the penne in the boiling salted water until al dente. Drain, rinse with cold water to prevent sticking and set aside.
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Now prepare the béchamel: Heat the milk in a small saucepan. Do not let it boil--as soon as tiny bubbles form at the edges, turn off the heat.
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Heat the butter in a medium sized, heavy saucepan over medium heat. When it has melted, add the flour and whisk until it turns light brown. Slowly add the warm milk, whisking the entire time. When all the milk has been added, whisk to remove any lumps and keep stirring until the sauce thickens enough to coat a spoon (stirring should leave an imprint in the sauce).
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Switch to a wooden spoon or spatula. Remove from the heat and stir in the salt and pepper. Stir in the cheese until melted. Then stir in the avocado puree and chopped cilantro.
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Taste the sauce--add more salt, pepper or lime juice if necessary.
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Mix in the reserved pasta. Dice the remaining avocado and mix it in as well.
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Serve garnished with cilantro and lime wedges immediately.