Heat oven to 180°. Cut a 2-3cm slit into the side of each chicken breast, stuff half the haggis into each breast and fold closed.
Heat oil in a frying pan on a medium-high heat and cook chicken for 2-3 mins each side. Remove from pan and place in a roasting dish in oven for 12-15 mins.
While the chicken cooks, add the onion to the pan and cook in the remaining oil until softened, add garlic and pepper and cook for 2 mins. Add cream and stock cube, bring to the boil, then simmer until the sauce has thickened; remove from heat. You can add a drop of boiling water to make a thinner sauce if you wish.
Remove the cooked chicken from the oven and serve with the sauce poured over, accompanied with creamy mashed tatties.