Spaghetti with Mussels, Lemon, and Shallots

INGREDIENTS

• Coarse salt
• 1 pound spaghetti
• 6 tablespoons extra-virgin olive oil, divided
• 3/4 cup diced shallots (4 large shallots)
• 1 teaspoon red-pepper flakes
• 1/4 cup chopped fresh flat-leaf parsley stems, plus 1 cup roughly chopped leaves
• 3/4 cup dry white wine
• 2 pounds mussels, scrubbed and beards removed
• 1/2 lemon, zested and juiced