If using 2 large chicken breasts, slice each piece of chicken into 2 cutlets by placing your hand on top of the chicken breast and running a sharp knife horizontally through the breast, slicing the top and bottom into two thin, even pieces.
Generously salt and pepper both sides of your chicken breasts.
In a deep 12-inch skillet with a lid heat oil over medium-high heat. Once hot, add chicken breasts to the pan and cook until browned, 3 to 4 minutes. Flip the chicken and repeat with the second side.
Remove skillet from heat, transfer chicken to a plate and set it aside.
To the same skillet, add butter. Melt butter over medium heat and then add the garlic and onions. Use a wooden spoon to stir, scraping the bottom of the pan to dislodge any cooked-on bits. Cook for 5 to 8 minutes, stirring occasionally, until the onions are wilted and beginning to soften.
Add the remaining ingredients, stir to combine, and bring to a simmer. Return the chicken to the pan and nestle it in the simmering liquid. Cover skillet, and cook for 5 to 10 minutes, until an instant-read thermometer inserted into the thickest part of the chicken breast reads 165°F.
Transfer chicken breasts to a clean plate or cutting board to rest and tent with foil to keep warm. Continue to simmer the sauce uncovered until the onions are softened to your desired level of doneness and the sauce reduces to a thicker consistency, about 3 to 5 minutes. Taste and add salt and pepper if needed.
Spoon sauce over chicken breasts and serve alongside rice, over egg noodles, or with mashed potatoes. Enjoy!!