"Grilled pork tenderloin marinated in a deliciously sweet blend of real maple syrup, Dijon mustard, and a hint of sesame oil. Because I have one step-daughter that only eats chicken or fish, I separated some of the sauce into two bags; one for pork and one for 2 chicken breasts. She gave me one of the best compliments she ever has in the last year for my cooking: 'It is fine-dining quality.'"
Whisk onion, maple syrup, Dijon mustard, garlic, horseradish, sesame oil, and black pepper together in a shallow bowl. Add pork to marinade and turn to coat. Cover bowl with plastic wrap and marinate pork, 8 hours to overnight.
Remove pork from marinade and pour marinade into a small saucepan. Cook marinade over medium-low heat until bubbling, about 5 minutes.
Heat olive oil in a skillet over medium-high heat. Cook pork in the hot oil, basting with marinade, until browned, 2 to 3 minutes per side. Reduce heat to low and cook pork, basting occasionally with marinade, until no longer pink in the center, 15 to 25 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).