Spray a small frying pan with oil and cook the onion and mushrooms over a medium heat for five minutes, or until well softened, stirring regularly. Tip into a heatproof bowl and leave to cool for five minutes.
Add the beef, thyme and lots of ground black pepper. Mix well and form into two balls. Flatten into burger shapes, each around 2cm/¾in thick.
Clean the pan and return to the hob. Spray with a little more oil and cook the burgers over a medium-low heat for 10 minutes, turning occasionally, until browned on the outside and cooked through inside.
Serve the burgers with lettuce, tomatoes and cucumber.
If you aren’t keen on beef, try making with minced chicken or turkey breast instead.