Drizzle the pork with olive oil, then season well with kosher salt and pepper on all sides.
In a nonstick pan, heat 1 tbs. olive oil. When warm, add diced onions and mushrooms and season with salt and pepper. Cook for 3-4 minutes, then add minced garlic and chopped thyme. Cook for another 1-2 minutes, then remove from heat.
Off the heat, sprinkle in the panko breadcrumbs.
Slice the pork tenderloin from end to end without cutting all the way through so it can open flat. Spread a thin layer of dijon mustard over the pork.
Stuff the tenderloin with the mushroom panko mixture. Take a few pieces of kitchen twine and tie the pork back together.
Place the tenderloin on a cookie sheet lined with foil. Roast in oven for 18 minutes, then check the temperature. Pork should be 135 degrees.
Remove the pork from the oven and let rest for 5 minutes. Remove twine with scissors, then slice! Serve warm.