Pat chicken dry inside and out with paper towels, then truss it securely with white kitchen cord.
Mix the butter and oil together, then brush chicken with this mixture until thoroughly and completely coated.
Preheat oven to 475°F.
Place chicken on its side on a rack in a shallow baking pan just large enough to hold the chicken.
Roast in the center of the oven for 10 minutes, then turn over onto other side, brush again with the butter-oil mixture, and roast for another 10 minutes.
Turn bird onto its back, reduce temperature to 400°F, baste with the butter-oil mix, sprinkle with salt and pepper, and roast for another 40 minutes (basting every 10 minutes with the butter-oil mixture and/or the juices in the pan).
To test for doneness, pierce the thigh with a sharp knife, if the juice that trickles out is pink, roast for another 5 – 10 minutes.
In a heavy 10 – 12" skillet, melt the butter over high heat, add the onionsandgarlic, lower heat to medium, and, stirring occasionally, cook uncovered for 3 to 5 minutes, or until the onions are soft but not brown.
Stir in the flour and mix into a paste, then pour in the chicken stock, bring to a boil over high heat, stirring constantly until the mixture thickens.