1. Peel onions, and cut off blossom and stem end.
2. Cut in half from top to bottom. 
3. Slice into 1/8 inch slices. 
4. Melt oil in a large (4 quart or larger) skillet over low-medium heat. (or work in batches using a smaller skillet)
5. Add onions, and let sweat, stirring occasionally. You can put a lid on the skillet during this phase so that the steam from the onions doesn’t escape and dry them out. Otherwise, just add a tablespoon or two of water if the onions start to stick to the pan. 
6. Continue cooking, stirring occasionally until the onions because soft, then clear, then begin to brown, while becoming softer, and get almost to the point of jaminess. This may take up to 30 minutes, depending on how deep your skillet is (a skillet with more surface area will caramelize faster, versus a smaller, deeper skillet, which will take longer).
7. When the onions become brown and jammy, add garlic, and stir in for about one minute. 
8. Stir in pepper, Worcestershire sauce, and thyme. 
9. Add sherry, and stir to deglaze the pan. Be sure to scrape the bottom of the pan to pull up any stuck bits. 
10. Finally, stir in beef broth, and bring to a simmer. 
11. Let cook down for about 15 minutes, then remove from heat. 

To Can: 

1. Prepare pressure canner, adding the inner rack, and required water to the bottom of the canner. 
2. Prepare jars and lids. 
3. Ladle soup into prepared canning jars, leaving 1/2 inch of headspace
4. Add salt to jars at a rate of 1 teaspoon per quart. This means 1/2 teaspoon per pint, 1/4 teaspoon per half pint, etc. 
5. Wipe rims of jars clean, then screw down lids and bands. 
6. Arrange in pressure canner so that the jars do not touch either each other, or the walls of the canner. 
7. Lock lid onto canner, and heat over medium-high heat with the steam valve open, until a steady stream of steam begins to escape. 
8. After ten minutes of steaming, close pressure valve and bring to ten pounds of pressure at sea level, or see this chart to convert to your elevation. 
9. Process quart jars for 75 minutes, or pint jars for 60  minutes.
10. Remove canner from heat, and let return to zero pressure naturally. 
11. Once pressure has come down, you may remove jars – carefully avoiding drafts and protecting yourself from super hot jars potentially breaking. Cover jars with a towel to finish cooling. 
12. After 24 hours, check for seals, wash jars if needed, and store in a cool place out of direct light. You may also remove the rings at this point if desired.