Bring stock to boil in a medium saucepan. Add farfalle and cook 8 to 10 minutes, or until just before the pasta would still be al dente (firm to the bite).
Meanwhile, heat olive oil in a small skillet over medium heat. Add breadcrumbs; cook and stir until golden brown. Remove from heat and transfer toasted breadcrumbs to a small bowl so they don't burn.
When the pasta is al dente, reduce heat and bring to simmer. Season to taste with salt and freshly ground black pepper. Add arugula and crack eggs gently into the broth.
After 3 minutes ladle soup into 2 bowls, 1 egg in each. Sprinkle toasted breadcrumbs, grated Parmigiano Reggiano, lemon zest (if using), and a generous drizzle extra virgin olive oil over each bowl and serve.