Heat BBQ to 350 °F.
In a large skillet, cook bacon for about 3 minutes, just long enough for some of the grease to melt off. Don’t let the bacon get crispy. Remove bacon from pan and set aside. Add a little olive oil to the bacon grease and sauté the onions until they are translucent. Add the garlic and spinach to the pan and cook until spinach is wilted. Remove from pan and mix in a bowl with the sundried tomatoes and feta. Season mixture with salt, pepper, and red pepper flakes. Mix well.
On a clean surface or cutting board, lay out flank steak. Cover the steak with a generous piece of plastic wrap (this keeps the meat juices from splattering all over your kitchen). With a meat mallet, pound out the steak. Flip the steak and repeat on the other side. The steak should end up ½ in thick or thinner, but still have enough integrity that it can still be stuffed and rolled up. Remove the plastic. With a sharp knife, start at the top left corner of the meat and run the knife blade to the bottom right corner, just scoring the meat, not cutting through it. Make parallel lines on both sides about an inch apart. Turn the steak in the other direction and do the same thing. It will make a little 1-inch diamond pattern on the meat surface. Turn the steak over and repeat to create a diagonal score pattern on both sides.
On a clean surface, lay out the bacon vertically with about ¼ inch between pieces to have it ready.
Baste the steak with olive oil and season with salt and pepper on both sides. Sprinkle the filling evenly over the meat. With the longest side of the meat closest to you, roll the meat away from you. Lay the meat seam side down on the strips of bacon. Wrap the meat with the bacon and use butcher string or toothpicks to secure.
Slice the meat between each piece of bacon. Place on the grill and cook for about 8 minutes on each side. Let rest for 10 minutes and enjoy with a glass of 2017 Mediterranean Vineyards Tempranillo.