Cook pasta according to package directions; drain. Return pasta to pan; cover and keep warm.
Meanwhile, for sauce, in a medium skillet heat oil over medium heat. Add 1/3 cup walnuts and garlic; cook and stir for 1 minute. Stir in flour, salt, and pepper. Gradually stir in evaporated milk. Cook and stir until slightly thickened and bubbly.
Pour sauce over pasta; toss gently to coat. Stir in arugula. Divide pasta mixture among warm shallow serving bowls. Top with 2 tablespoons walnuts and cheese.