Preheat oven to 350 degrees.
Place the pork loin in the baking dish, fat side up. Season both sides with salt and pepper.
Combine mustard, maple syrup, orange juice, red wine vinegar, and 1 minced garlic clove together. Pour the sauce over the meat. Add rosemary sprigs and orange slices to the baking dish.
Bake for 40 minutes and check the temperature, it is cooked through at 160 degrees.
While the pork loin is roasting, saute the onions and mushrooms over medium heat in olive oil. Add garlic and the white wine and let it cook until the wine has reduced. Add the onions and mushrooms back into the pan.
When the pork loin is done, remove it from the oven and let it rest for 10-15 minutes on a plate. Add 1 Tbsp flour to the leftover liquid in the baking dish and whisk it until it thickens (Do this step while the liquid is still hot.)
After 15 minutes, slice the pork and put it back into the dish and spoon the sauce over the top. Top with mushrooms, onions, and chopped rosemary.