Mix rosemary, sage, chopped garlic, mustard, salt, and pepper in a small bowl. Brush the mixture over the fillet and coat with flour.
Heat butter in a large pan over high heat. Sear fillet for 3-4 minutes on each side until browned. Pour in white wine to deglaze the pan. Reduce heat to medium-low and simmer for 15-20 minutes until pork reaches 63°C (145°F) internally.
Stir in chopped parsley and crème fraîche. Season to taste with vegetable stock powder.
In a separate pan, sauté chopped onion (or shallots) over medium heat for 3-4 minutes. Add mushrooms, vegetable stock powder, salt, pepper, and heavy cream. Cook over medium heat for 5-7 minutes until mushrooms are tender.
While the pork cooks, prepare wild rice according to package directions.
Arrange pork medallions, mushroom mixture, and rice on plates. Garnish with fresh parsley.