Fry the bacon strips on both sides until crispy. Blot the bacon with paper towels to remove excess grease and chop into small pieces. Combine the bacon bits, bacon grease, softened butter, paprika, parsley, honey, salt, and pepper. Spoon the bacon butter mixture onto a piece of plastic wrap, form the mixture into a log, tightly seal the plastic wrap, and refrigerate until hardened.
Peel the potatoes, cut them in half, and boil in lightly salted water. Add the butter, egg yolk, salt, pepper, and nutmeg to the boiled potatoes and mash until smooth. Fill a piping bag with the mashed potato mixture and create spiral mounds on a baking tray lined with parchment paper. Brush the egg wash over the duchess potatoes, transfer the baking tray to a preheated oven, and cook at 350°F for 15 minutes.
Heat some olive oil in a pan over medium heat and fry the mushrooms. Stir in the spinach, heavy cream, salt, pepper, nutmeg, and parmesan cheese and bring to a simmer.
Heat some olive oil in an oven-proof pan over medium heat and sear the beef tenderloin on each side. Reduce the oven temperature to 275°F, transfer the pan to the oven, and cook for 30 minutes.
Take the hardened bacon butter roll out of the fridge, remove the plastic wrap, and cut the roll into 5 even slices. Cut the beef tenderloin into 5 even slices and serve each slice with a helping of duchess potatoes, spinach parmesan sauce, and bacon butter.
