Place the roast in the freezer for 20 to 25 minutes, until it feels firm when you squeeze it but not rock solid. This simple precaution will make your roast much easier to slice.
Slice the roast into 20 thin slices of roughly 1/4" each; pound each slice with a meat mallet until it becomes almost paper thin.
Lay two pieces of plastic film side-by-side on your work surface in order to form a large rectangle. Arrange the paper thin slices of meat over that plastic film, creating as big a rectangle as you possibly can while making sure that all the slices of meat overlap.
Sprinkle the meat with the salt, pepper and thyme and then arrange the red bell peppers over the meat in a single layer. Top with the baby spinach leaves and finally, sprinkle with the shaved parmesan.
Starting at one of the shorter ends, ideally the one where the meat is tucked underneath the other rows, start rolling the meat, as tightly as you can, using the plastic film to support and form the roll as you go. Wrap the roll tightly with one or two additional layers of plastic film and place the roast in the refrigerator to chill and firm up for at least 3 hours, or up to overnight.
Tie the roast with butcher's twine in 8 different locations, about every two inches or so, and then trim off the excess twine.
Next, using a sharp knife, slice the roast between each of the strings, thus creating 8 beef pinwheels.
Heat a skillet over really high heat and add a liberal amount of your healthy cooking fat of choice. When the pan is nice and hot, add the pinwheels and cook for about 3 minutes per side.
Remove the cooked pinwheels to a plate, tent lightly and allow to rest for about 5 minutes before serving.