Season trimmed tenderloin generously with salt and pepper. In a sauté pan over medium-high heat, heat oil and brown pork on all sides. Remove from pan and place on a baking sheet.
Place pork in oven and cook until it reaches 145º F (about 10 to 14 minutes). Let rest before slicing.
Meanwhile, place the sauté pan back on the stovetop over medium heat. Add the butter and sauté the onions. Season with salt and pepper and cook until onions soften and start to turn golden.
Add apple cider and vinegar. Raise heat to medium-high and let reduce by half.
Add the mustard and the maple syrup and adjust seasoning if necessary.
Slice the tenderloin against the grain and serve with the sauce.