Preheat the oven to 400°F.
In a large saucepan, heat the milk, cream, lemon zest and garlic. Bring to a simmer for 30 minutes.
Meanwhile, in a second small saucepan, melt the butter. Add the flour and cook, stirring, for 10-15 minutes, until golden. Cool.
Little by little, add the cooked roux and parmesan to the milk and cream mixture over medium-low heat, whisking well between each addition. Continue to simmer for 10 more minutes, or until thickened. Stir in the lemon juice and remove from the heat. Fold in the parmesan cheese.
Place the sausages on a baking sheet and roast until crisp, 10-12 minutes.
On a second baking sheet, toss the broccoli rabe and leeks with the oil, salt and pepper. Roast until crisp and fork-tender, 10-12 minutes.
In a blender, combine all of the pistachio pesto ingredients until smooth. Taste and adjust seasoning.
Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions. Drain the pasta, return to the pot, and toss with the alfredo sauce, roasted peppers, leeks, sausages and piquillo peppers.
Divide between plates and top with ricotta salata and pistachio pesto if you like.