Place the chicken cubes in a bowl. Add the minced garlic and pepper. Mix well. Cover and keep in the fridge for an hour.
Heat the butter and olive oil in a wide shallow pan — wide enough to contain the chicken cubes in a single layer. The heat should be very high.
Place the chicken in a plastic freezer bag. Add the flour. Shake to coat each piece of chicken with flour. It is the flour that will thicken the sauce later and make it stick well to the chicken.
When the olive oil and butter are hot, add the floured chicken, spreading the pieces so that every piece touches the oil. Do not stir for a minute or so to allow the underside to brown. Keep the heat very high.
Stir. Cook for a few minutes, with occasional stirring, until the chicken changes color and a light crust forms.
Add the mushrooms, cook just until softened, stirring occasionally.
Pour in the Worcestershire sauce and liquid seasoning. Stir briskly; the sauce should thicken quite fast.
Continue cooking until the liquid seasonings have been soaked up by the chicken and mushrooms.