Warm a deep frying pan or shallow casserole dish over a medium heat for 2 mins. Add ½ tbsp oil and the chicken breast mini fillets. Fry for 6-8 mins, turning once, till the chicken is golden brown.
While the chicken is cooking, peel and finely chop the onion. Peel and dice the carrot. Trim the dry ends off the celery and finely slice it.
Lift the chicken breast mini fillets out of the pan and pop them on a plate. Add the chopped veg to the pan with 3 tbsp water and some salt and pepper. Fry for 5 mins, stirring now and then, till softened and a little golden.
While the veg fries, finely chop the tomato. Peel and crush or grate the garlic. Strip the leaves off the rosemary sprigs and roughly chop them. Drain and rinse the butter beans.
Stir the tomato, garlic and half the rosemary leaves into the pan. Crumble in the stock cube. Tip in the butter beans. Pour in 500ml boiling water and stir to mix.
Slide the chicken breast mini fillets back into the pan. Cover, turn the heat up and bring to the boil. When boiling, turn the heat down and simmer for 10-15 mins till the chicken breast mini fillets are tender and cooked through.
While the chicken simmers, slice the thick cores out of the kale leaves. Finely shred the leaves.
Add the kale to the pan and push it down around the chicken with a wooden spoon. Cook for 3-4 mins till the kale is just tender. Taste the casserole and add more salt and pepper if you think it needs it. Serve in warm bowls with the rest of the rosemary sprinkled over the top.