Boil salted water and cook bow tie pasta until al dente, about 8-10 minutes.
Reserve 3/4 cup pasta water before draining.
Toss drained pasta with 2 Tbsp reserved water and set aside.
Melt butter in a large saucepan over medium-high heat.
Add spring onion and sauté for 3 minutes until softened.
Add garlic and cook for 1 minute until fragrant.
Sprinkle flour over onions and garlic, whisking constantly for 1 minute.
Slowly pour in heavy cream while whisking to avoid lumps.
Whisk in mascarpone, Parmesan, and ground mustard until smooth.
Add 1/4 cup reserved pasta water to thin the sauce.
Stir in ham and peas and heat through for 3-5 minutes.
Remove from heat and stir in lemon zest, lemon juice, 2 Tbsp parsley, nutmeg, salt, and pepper.
Pour sauce over pasta and toss until well coated.
Add more reserved pasta water if needed for desired consistency.
Taste and adjust seasonings.
Garnish with remaining parsley and extra Parmesan before serving.