Cauliflower Spaghetti Alfredo with Shrimp

Name: Cauliflower Spaghetti Alfredo with Shrimp
Ingredients: Kosher salt; 1 pound spaghetti; Olive oil; 1/2 yellow onion, finely diced; 2 cloves garlic, minced; 1 head cauliflower, cut into florets; 4 cups whole milk, plus more if needed; 1 tablespoon kosher salt; 1/4 cup grated Parmesan, plus more for serving; 1/4 cup coarsely chopped fresh parsley, plus more for serving; Juice and zest of 1 lemon; 1 pound large shrimp, peeled and deveined; Kosher salt and freshly ground black pepper; Lemon wedges, for garnish