1 Start by making the pesto. Put the parsley, cheese, garlic, salt and walnuts into a food processor. Pulse to combine. Turn the machine on again and slowly pour in the olive oil, just to combine. Reserve.
2 Cook the bacon in a large pan — you will be searing the roulade in this later, so it needs to be wide — over medium-low heat until it is about half-cooked. You want it cooked, but still limp. Do not crisp it up or it will break when you try to wrap it inside the roulade. When the bacon is ready, set it aside on paper towels.
3 Place heavy duty plastic wrap (or two layers of plastic wrap) on a large work surface and place the flank steak on it. Cover with more plastic wrap. Using a rubber mallet, the flat side of a meat mallet or an empty wine bottle, pound the flank steak until it is 1/2 inch thick or thinner. Flip the meat from time to time to pound everything evenly. Once the meat is as thin as you want it, if you have a meat mallet with a tenderizing side (the pointy side), remove the plastic wrap and pound this on both sides for a minute or two. If you don't have a meat mallet, you can skip this step.
4 Look at your steak: You will be rolling it up with the grain of the meat facing side to side. You do this because when you slice it later, the beef will be more tender when you cut across the grain. Arrange the meat until the grain faces side to side, and if it is not squarish or rectangular, cut it to fit. Sprinkle on some salt and black pepper.
5 Spread a thin layer of pesto on the meat, leaving about 1/2 inch free on all sides of the meat. Lay down the bacon against the grain of the beef. Preheat the oven to 350°F.
6 Carefully roll the roulade up tightly, as you would a carpet. If you want, cut off any bacon that is extending beyond the steak. Tie off the meat with 6 to 8 lengths of string, each about an inch or so apart. Sear the tied roulade in the pan with the bacon fat. You want to quickly brown the surface, not cook the inside of the roulade.
7 Put the roulade in a roasting pan on a rack, seam side down. If you don't have a rack, improvise with celery stalks. Roast this for 20-25 minutes, or until the interior of the meat is 130°F. (Note that the ends of the roulade will be far hotter than the center — so always test the temperature from the center of the roulade.)
8 Remove the meat from the oven and let it rest for 10 minutes before slicing. Slice the roulade so each serving is wrapped in string. You can either let everyone cut their own string at the table, or cut it yourself and secure the roulades with toothpicks if you want. Serve with lemon wedges to add a little tartness to the dish.