Heat the oil with the garlic in a shallow casserole and cook until fragrant, about 1 minute. Add the chicken breast, fry over a medium-high heat until golden. Flip the breast then sear the bottoms. Season with salt and pepper to taste. Add hot chicken broth and simmer gently for about 25 minutes turning the breast from time to time until chicken is fully cooked.
If the sauce is getting too dry, add a splash of water/broth and stir gently. Wrap chicken breast in foil and let it rest for 10 minutes then slice it into stripes and put it back in the casserole. In the meantime, cook the Fettuccine according to package instructions and drain, reserving ½ cup of the pasta cooking water. Add the Fettuccine to the casserole and stir gently, adding pasta water a little at a time if you need to loosen the sauce.
Pour Portobello Mushroom Sauce also in the casserole with the striped chicken and heat gently on low heat. Divide pasta among plates and serve immediately.