Preheat the olive oil in a large frying pan over medium high heat. Add the chicken in the pan and season with salt and pepper. Cook to sear on both sides, about 5 minutes per side.
Add in the onions and cook 5 minutes. Add in the mushrooms and stir to combine. Cook for about 10 minutes while stirring the ingredients together often.
Add in the garlic once the mushrooms have browned and reduced in size. Season with salt and pepper. Stir to combine, and cook for 3 minutes.
Add in the lemon juice and scrape the pan with a wooden spoon to remove any brown bits from the pan and incorporate them into the sauce.
Stir in the stock, parsley, and milk and bring to a simmer. Cook for about 10 more minutes until the chicken is cooked through.