Preheat oven to 375°.
Combine first 4 ingredients in a small saucepan over medium-high heat. Stir in 1/2 cup broth; bring to a boil. Reduce heat, and simmer until reduced to 1/4 cup (about 7 minutes). Strain broth mixture through a sieve into a bowl; discard solids. Stir in salt and pepper.
Separate onion slices, and place them in an even layer in the bottom of a shallow roasting pan.
Remove and discard giblets and necks from hens. Remove skin; trim excess fat. Working with 1 hen at a time, tie ends of legs together with cord. Lift wing tips up and over back; tuck under hen. Set hens, breast side up, on top of onion slices; brush tops and sides of hens with broth mixture. Bake at 375° for 30 minutes. Add remaining 1/2 cup broth to pan. Bake an additional 10 minutes or until thermometer registers 165°. Remove from oven, and transfer hens to a cutting board or work surface. Keep warm.
Place a zip-top plastic bag inside a 2-cup glass measure. Strain broth mixture through a sieve into zip-top bag; discard solids. Let reserved drippings stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain drippings into a small saucepan, stopping before fat layer reaches opening; discard fat. Bring drippings to a boil over medium-high heat. Cook until reduced to 3 tablespoons (about 5 minutes).
Split hens in half lengthwise. Place one hen half on each of 4 plates; drizzle each serving with 2 teaspoons drippings.