Chop the chicken breast mini fillets into bite-size chunks. Warm a deep frying pan or a shallow casserole dish for 2 mins, then add ½ tbsp oil and the chicken. Fry for 5-6 mins, turning once or twice, till the chicken is browned all over.
While the chicken fries, trim the spring onions' roots and 1cm off the green tops. Finely slice the spring onions. Scrub the potatoes and chop them into chunks to match the chicken.
When the chicken has browned, scoop it out of the pan and pop it on a plate. Add the spring onions and potatoes to the pan with ½ tbsp oil and a good pinch of salt and pepper. Fry for 5 mins, stirring now and then, till the onions and potatoes have picked up a little colour and started to soften.
While the veg fry, peel and crush or grate the garlic. Crumble the stock cube into a heatproof jug and stir in 300ml boiling water to dissolve it.
Stir the garlic and 1 tbsp mustard into the pan. Cook and stir for 1 min. Add the chicken back into the pan and carefully pour in the chicken stock (the pan may splutter as you add the liquid). Pop a lid on the pan (or use a baking tray if you don't have a lid) and bring the chicken and veg to the boil. Once it's bubbling, turn the heat down and simmer for 10 mins.
While the chicken cooks, pop the broad beans out of their pods (see our tip below about double podding). Pick the sage leaves off the stems and finely slice them.
Add the broad beans to the pan with half the sage and simmer, without the lid on the pan, for 4-5 mins. The chicken should be cooked through (no pink) and the vegetables just tender.
Turn the heat to low and slowly stir the crème fraîche into the pan 1 spoonful at a time. Gently warm through for 2 mins (don't heat it too quickly, as the crème fraîche may split). Taste and add a pinch more salt or pepper if you think it needs it.
Ladle the chicken, vegetables and the creamy sauce into warm bowls and garnish with the last of the sage to serve.