Preheat the oven to 400°F (200°C).
Prepare the filling by heating olive oil in a skillet over medium heat.
Add garlic and mushrooms, sautéing until they soften.
Stir in spinach and sun-dried tomatoes, cooking until the spinach is wilted.
Remove from heat and fold in ricotta and Parmesan cheese.
Season the filling with salt and pepper to taste.
Prepare the beef tenderloin by trimming excess fat and tying it with kitchen twine.
Cut a pocket in the beef tenderloin and stuff it with the filling.
Sear the tenderloin in a hot skillet until browned on all sides.
Transfer the tenderloin to the oven and roast for 25-30 minutes until desired doneness.
Remove from the oven and let it rest for 10-15 minutes.
Slice and serve, garnished with fresh herbs.