Cook the bow ties in a large pot of lightly salted boiling water until al dente. Reserve ½ cup of pasta water, then drain. Place the pasta back in the large pot and toss with 2 tablespoons reserved water.
Melt the butter in a large saucepan over medium high heat, then saute the spring onion for 3 minutes. Add the garlic and saute one minute, until fragrant.
Sprinkle the flour on the veggies, whisking constantly, and cook for one minute to get rid of the raw flour taste. Gradually add the heavy cream while whisking until smooth. Continue to whisk in the mascarpone, Parmesan, and the dry mustard to get a smooth sauce. Add ¼ cup of the reserved water.
Add the ham and peas, and heat through, about 3 to 5 minutes. Remove from the heat. Stir in the lemon zest, lemon juice, parsley, nutmeg, salt, and pepper.
Pour sauce over the pasta and mix well to coat. Adjust the seasonings to taste.
Garnish with reserved parsley and grated Parmesan cheese. Serve warm.