Place chicken in a single layer on large cutting board. Use paper towels to pat dry chicken. Cover chicken with cling wrap and pound to an even 1/2" thinness. Generously sprinkle chicken on both sides kosher salt, freshly ground black pepper, and garlic powder.,

Heat 2 TB oil in large skillet over medium high until sizzling. Pan fry chicken 2 pieces at a time, until just cooked, 2-3 minutes per side until nicely browned. Remove from heat, and let sit 3 minutes. Cut chicken into strips and keep warm. Begin boiling large pot of generously salted water while you make the sauce.,

Mix milk and flour in bowl. Sauté chopped garlic in 1 TB oil in a large skillet over medium heat until fragrant and softened. Reduce heat to low. Slowly add milk mixture and bring to a low boil while stirring. Simmer and stir at least 3 minutes or until sauce thickens. Add parmesan cheese. Stir until cheese is melted. Stir in chopped spinach, if using. Add salt and pepper to taste. Turn heat off and keep covered.,

Cook fettuccine in boiling water to al dente. Reserve 1 cup of pasta water, and drain the rest. Do not rinse pasta. Transfer drained pasta back into pot and toss with sauce. Add as much reserved pasta water as needed until desired consistency – I add all of it, as it tends to thicken with time. Gently toss in freshly chopped basil. Serve immediately with sliced chicken and additional parmesan.