- Rinse chicken; pat dry.
- Reserve 2 tablespoons herb butter for gravy.
- Slide hand under skin of chicken breast to loosen skin from meat and place in slices of the herb butter.
- Spread remaining herb butter over outside of chicken.
- Truss chicken.
- Place the chicken in a flameproof roasting pan.
- Slice the onion into large pieces and scatter around the chicken.
- Arrange birds in a flameproof roasting pan.
- Roast bird in middle of oven until an instant-read thermometer inserted in thickest part of a thigh (be careful not to touch bone) registers 170° to 175 °F.
- It will take roughly 55 – 65 minutes for a four pound chicken and 8 minutes more per every pound over that.
- Lift chicken and tilt, emptying any juices from cavity into roasting pan.
- Remove the chicken to the platter, cover loosely, and make the following sauce.
- Add vermouth to pan.
- Place pan over high heat; bring vermouth to boil, scraping up any browned bits and onions.
- Strain the mixture into cup with pan juices.
- Spoon fat off.
- Pour back into the pan.
- Bring to a simmer; melt reserved 2 tablespoons herb butter in heavy medium saucepan over medium-high heat.
- Add flour or Wondra; whisk until smooth.
- Serve with gravy.