Heat the olive oil in a pan over high heat.
Sprinkle the chicken on all sides with salt and pepper. Sear chicken on both sides in a hot pan until browned (chicken will not be cooked through).
Transfer the chicken to a slow cooker.
Lower the heat to medium, stir in the onions, cook until browned. Add the mushrooms, rosemary and sage, cook until darkened and softened.
Sprinkle with flour, stir until combined. Add the chicken broth, beer, and brown sugar, scraping to deglaze the pan. Pour the mushroom mixture over the chicken, stir to combine.
Cook in a slow cooker on high for 4 hours or until chicken shreds easily with a fork. Salt and pepper to taste. (if sauce doesn't thicken as much as you like, add to a pan over medium high heat, simmer until thickened.)
Serve over rice or pasta.