Preheat oven to 375°F (190°C).
Place beef tenderloin on countertop to come to room temperature.
Combine onions, green onions, pearl onions, garlic, thyme, butter and canola with a small amount of salt and black pepper. Put in a pan (ideally cast iron) and place in the preheated oven.
Roast onions until lightly golden and very tender, approximately 30 minutes.
Leaving as much butter and oil in the pan as possible, transfer the onion mixture to a bowl and use a fork to lightly mash the garlic, almost like a paste. Mix garlic paste throughout onion mixture.
Using the same pan you cooked the onions in, with butter and oil reserved, place the pan on the stove top over medium-high heat.
Season tenderloin with salt and pepper and then place it in the hot pan. Sear for 1-2 minutes per side, depending on your preferred degree of doneness. (For a medallion with a 3cm thickness, a 2 minute sear per side will give you a nice medium centre.)
Spoon onions over tenderloin, spoon vinegar over onions and cover with a piece of Albert’s Leap Le Bon Secret.
Place pan back in oven for 1 minute until cheese is just melted.
Remove from oven and let rest for 3-5 minutes before serving.