Mix flour and pepper in a shallow dish . Dredge chicken pieces in flour mixture to coat. Shake off excess.
In a large skillet, heat 2 tablespoons vegan butter substitute with the olive oil. Add chicken and brown lightly, about 3 minutes per side. Do not crowd pan; cook in batches if necessary. Remove from pan.
Add onion and garlic to pan and saute until tender, 3-5 minutes. Add mushrooms and cook until they are lightly browned, 5 minutes.
Return chicken to pan without crowding. Stir in 3 tablespoons of Marsala wine and stock. Bring to a boil and simmer about 10 minutes. If you have more chicken, remove first batch to a clean plate, and saute second batch. You do not need to add more wine or stock.
When all of the chicken has been cooked, remove it to a clean plate and whisk remaining vegan butter into pan drippings. Add remaining two tablespoons of Marsala wine and scrape pan with a wooden spoon to stir up the browned bits.
Pour remaining sauce in pan over chicken before serving.
This recipe was adapted from 365 Ways to Cook Chicken