Peel and mince garlic cloves and red onion.

Zest lemon, then squeeze one half until you have 1–2 tablespoons of lemon juice.

Bring a large saucepan of water to boil, add a couple of teaspoons of salt, and cook pasta to al dente according to package directions. Reserve ½ cup of cooking liquid before draining.

While pasta is cooking, sauté butter in a 12-inch skillet over medium-low heat, then add garlic and onions.

Sauté until garlic and onions are softened, but not browned, about 3-5 minutes.

Add mascarpone cheese and cream, stirring to melt the cheese. Bring to a simmer over medium heat, stirring frequently, until sauce thickens (about 5 minutes).

Stir in 1 tablespoon lemon juice and 1 tablespoon zest.

Taste and add salt, pepper, and more lemon juice as needed.

Stir in drained pasta and arugula.

Toss until pasta is heated through, arugula is wilted, and sauce coats everything (about 1 minute).

Serve immediately.