Cut chicken breasts into bite-sized pieces. Pat dry. Sprinkle with 1/4 teaspoon salt and about 1/8 teaspoon freshly ground pepper.

Cook the bacon, crumble it, and set aside.

Sauté the chicken breast in a Tablespoon or two of olive oil until browned and cooked thoroughly. Cook the chicken in batches to avoid crowding the pan and ensure even browning.
Transfer cooked chicken to a bowl or plate while you cook the other ingredients.

Sautée the mushrooms in a few teaspoons of olive oil until their liquid has cooked off and they are lightly browned.
Transfer the mushrooms to the same bowl as the chicken.

Sauté the diced onion in a few teaspoons of olive oil until it is soft and translucent. Add the garlic, and sauté for one minute more.
Transfer the onion and garlic to the same bowl as the chicken and mushrooms.

Add about a quarter of the warm milk to the pot and whisk vigorously until smooth. Slowly add the remaining milk, whisking constantly over low heat until it is smooth.

Continue cooking over low heat until the mixture comes to a boil. Do not stop whisking during this process! Once boiling, reduce the heat and allow the sauce to cook for 2 to 3 minutes longer.
It should thicken and coat the back of a spoon. If you drag your finger through the sauce on the spoon, it should leave a distinct trail as shown in the photo.

Add the Parmesan cheese and whisk again to fully incorporate.

Taste and add salt and pepper as necessary.

Pour the sauce over the chicken mixture and stir well to fully coat everything.