Use a stainless steel pan instead of a nonstick here, if possible. A stainless surface will better collect fond (also known as browned bits) from the pork, which is then deglazed to lend rich flavor to the mushrooms and onions as they cook.
How to Make It
Heat oil in a large high-sided skillet over medium. Sprinkle pork with 1/2 teaspoon salt and 1/2 teaspoon pepper. Add pork to pan; cook, turning occasionally, until browned on all sides and a meat thermometer inserted into thickest portion registers 145°F, about 15 minutes. Remove pork from pan; keep warm.
Add mushrooms, onions, thyme, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper to pan; cook, stirring and scraping pan to loosen browned bits from bottom of pan, until vegetables are soft, about 7 minutes. Cut pork crosswise into thin slices; serve with mushrooms and onions.