Trim the tenderloin and remove a bit of the tail part where it is thinner. Cut the tenderloin in half. Butterfly each piece lengthwise about 1/2-inch, then do the same the opposite way. This will give you two flat pieces.
Lay half the spinach on each piece of meat. Add half the hazelnuts,then half the blueberries to each piece; then split the cream cheese in two and with your hand; make flat discs to place on top of the ingredients. Then roll tenderloin back up nice and tight without losing any of the stuffing.
Rub Dijon mustard on the outside; sprinkle with salt and pepper. In a frying pan, heat the olive oil and sear the tenderloins for two minutes on each side.
Place on a baking sheet lined with parchment paper and bake in a preheated 350F oven for 20 minutes. Remove from oven and let rest for 10 minutes, cut 4 times on the bias and serve.
This dish can be accompanied with a warmed blueberry jam.