Cut off the top of garlic head and drizzle with olive oil. Bake for 20-25 minutes.
While the garlic is roasting, slice onion thinly.
Lay your ribeyes on a plate and spread a thin layer of head country on both sides. Set aside.
In a skillet on low heat, add your onions and cook stirring frequently. You will want to caramelize the onions which will take about 30 minutes.
When the garlic is finished, remove from oven and squeeze garlic into a bowl. Mash and then add mayonnaise and Worcestershire sauce and mix. Place in refrigerator.
Butter both sides of toast and either toast in the oven or toast in a skillet. Toast both sides of bread lightly.
Grill Ribeyes to desired doneness.
Spread aioli to both sides of bread then add caramelized onions and steak. Serve.
Notes
My picture shows a medium well sandwich which was cooked over direct heat of 425 degrees for 1-2 minutes per side.
The timing may be different for you depending on your oven. I have my garlic roasting while I work on my onions. Once I am close to finishing the onions, I start my toast.
You may cook the aioli ahead of time. An aioli will last up to 7 days in the refrigerator.