Heat a 12" skillet on medium heat; add butter and cook until butter foams, giving the pan a good occasional swirl. Once butter foams, keep a close eye on it, the color will change quickly. As soon as it starts to turn a light brown color, lower the heat to the lowest setting. Add walnuts, sage, and garlic and cook for 1 minute. Add spinach and reserved hot water to the pan, season with salt and pepper; stir and combine well (about 2 minutes). Add ravioli and finish with fresh parmesan and crushed red pepper (if using). Enjoy!