Preheat the oven to 160℃/320F fan, 180℃/350F, Gas Mark 4.
Pat the beef dry with kitchen towel, then rub the beef all over with 2 tbsp olive oil, dried thyme and a good pinch of salt and pepper.
Heat a large frying pan over a medium high heat and sear the beef until it’s a lovely dark brown all over.
Spread the onions and the garlic cloves out on the base of a roasting tin and drizzle with 1 tbsp of the olive oil. (If it’s hob safe, making the gravy later will be easier.)
Sit the beef on top of the vegetables and cook for 50-60 minutes for medium rare.
Remove the beef from the roasting tray, place on a warm plate and cover with foil to rest for 20 minutes while you make the gravy.
To make the Gravy:
Put the roasting tin on the hob over a medium heat. (Transfer the contents of the roasting tin to a saucepan if your dish can’t go on the hob. Be sure to scrape out all of the good, tasty bits!)
Cook the onions and garlic over a medium heat for a few minutes until they are golden brown, then stir in the plain flour.
Gradually pour in the beef stock, little by little, while scraping up any bits that are stuck to the bottom of the pan.
Simmer for 5-10 minutes until you have a lovely thick gravy, then strain through a sieve. Pop the strained gravy back in the pan and stir in the mustard and season to taste with salt and pepper.
Slice the beef and serve with the gravy.
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