Preheat oven to 400 degrees F. Line 2 baking sheets with parchment paper and grease each pan with 1 tbsp. olive oil. Thinly slice 3 zucchini into rounds and place, without overlapping, on the baking sheets. Drizzle each pan with an additional tbsp. of olive oil and ½ tsp. salt.
Bake until golden brown, about 30 minutes. Remove from the oven and set aside.
Bring a large pot of salted water to a boil.
Dice the remaining zucchini for the pesto.
Heat 4 tbsp. olive oil over medium heat in a large skillet. Add the onion, sauté for 3 minutes.
Stir in the diced zucchini, and a pinch of salt. Cover and cook over low heat, stirring occasionally, until the zucchini is soft, about 20 minutes.
Transfer the mixture to a food processor or blender and add the garlic, basil leaves, and grated cheese. Process until smooth and creamy. Taste and adjust seasoning, if necessary. Pour the pesto back into the skillet.
Add pasta to the boiling water and cook until al dente. Drain and add to the skillet with the zucchini pesto. Toss to coat the pasta. Stir in a ladle of pasta cooking water to loosen the sauce as needed.
Stir in the roasted zucchini slices, reserving a few slices to top the pasta when serving. Top with a drizzle of olive oil and a few extra basil leaves, if desired.
Ladle the pasta into bowls and top with the reserved roasted zucchini. Serve with extra grated cheese at the table.
Notes
To freeze zucchini pesto: If you have a large zucchini crop and are wondering what to do with it, you may freeze zucchini pesto for up to 3 months. Pasta suggestions:Use medium-length pasta shapes such as elicoidali; rigatoni; penne; farfalle or gemelli. Long pasta such as spaghetti or linguine also work well.