Heat the oil in a large frying pan over medium-high heat. Add the onions and sauté over medium heat for about three minutes.

Add the sugar and reduce heat to medium-low. Allow the sugar to caramelize with the onions for about 10 minutes, until the onions are light brown.

Add the beer and cook until liquid is reduced by half.

Dissolve the cornstarch in the orange juice and add to the pan. Let cook until the mixture thickens, and add remaining ingredients. Stir until combined. Cool and refrigerate up to four days.

Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).

Arrange orange slices in a pan and place butterflied Cornish hens on top. Brush hens with sauce and cover the pan. Bake for about 45 minutes. Uncover, baste with sauce again, and bake uncovered for 20 minutes.