Tip the buckwheat into a sieve and give it a good rinse. Tip it into a pan. Pour in 350ml boiling water. Add a pinch of salt. Cover and bring to the boil. Turn the heat right down and simmer for 15 mins till the buckwheat is tender.
While the buckwheat cooks, peel and slice the red onion. Halve the pepper and scoop out the seeds and white bits. Roughly chop it. Break the broccoli into small florets. Peel and crush or grate the garlic.
Warm a wok or deep frying pan over a medium heat for 1 min. Add ½ tbsp olive oil and the beef stir-fry strips. Stir fry for 3-4 mins till the beef is browned all over. Scoop the beef out of the pan onto a plate.
Add another ½ tbsp oil to the pan with the onion and pepper. Season with salt and pepper. Cook and stir for 5 mins to just soften the veg. Stir in the garlic.
Drain the lentils and add them to the pan. Return the beef to the pan. Pour in 250ml boiling water. Crumble in the stock cube and stir to mix. Pop on a lid and bring it to the boil, then add the broccoli florets and simmer for 8-10 mins till the broccoli is just cooked.
While the beef braise cooks, finely grate the zest from the lemon. Juice it. Whisk the lemon zest, juice and tahini together to make a smooth sauce.
Stir the tahini sauce into the pan. Taste and add more salt and pepper if you think it needs it.
The buckwheat should be cooked by now. Drain it and divide between 2 warm bowls. Ladle the beef over the top and serve.