In a large bowl, mix the tenderloin, the garlic, the red onion, 3-4 tbsp of olive oil, 1-2 tbsp of red wine vinegar, some thyme, some rosemary, and mix with your hands.

Wrap the bowl and put it into the fridge. Leave the marinade in the fridge for 3 hours.

In a hot large non-sticking pan, add the tenderloin along with the ingredients of the marinade, and sauté for 1 minute on each side, without stirring, until the meat is nicely coloured, crispy and tender inside.

Arrange some tomato slices and tzatziki on the bottoms of each hamburger bun and place the gyro on top. Top with grated cheese of your choice (optional).

Place the top bun on each burger and serve with French fries.