Bring large pot of salted water to a boil for pasta. Toast walnuts in large nonstick skillet over medium heat, shaking skillet frequently, until fragrant and lightly browned, 3 to 5 minutes. Transfer toasted walnuts to plate and set aside.

Add 1 tbsp olive oil to same skillet over medium-low heat. Add leeks and cook until softened, 3 to 5 minutes. Add garlic and Dijon, stir to combine, and remove from heat. (TIP: Lower heat if leeks begin to brown.)

Add pasta to boiling water and cook until al dente, 8 to 10 minutes. Reserve ⅓ cup pasta water, drain pasta, and return to pot, off heat.

Add melted leeks, reserved pasta water, butter, and herb garlic cheese to pot with pasta. Stir until butter and cheese melt and evenly coat pasta. Season to taste with salt.

Add lemon juice and arugula to pasta and toss to combine. Divide mafaldine between plates. Top with toasted walnuts, tarragon, and a pinch of pepper. Serve with lemon wedges. Bon appétit!

For 4-serving meal, double all ingredients. You may need to cook in batches, in larger cooking vessels, or for longer cook times. Follow visual cues in 2-serving recipe.