Kosher salt
penne pasta
unsalted butter or extra-virgin olive oil
asparagus, trimmed, cut into 2" pieces
large shallot, thinly sliced into rounds
cloves garlic, thinly sliced
Finely grated zest of 1 medium lemon (about 2 tsp.), divided
Freshly ground black pepper
(5.3-oz.) container plain Greek yogurt (preferably 5% fat or more)
feta (about 1 c.), crumbled into small pieces, plus more for serving
chopped fresh dill, divided
coarsely chopped toasted walnuts