Add butter to a large skillet. Melt over medium/low heat. Add onions and stir to evenly coat. Cover with lid and let cook for 10 minutes, stirring occasionally.

Add 8 ounces of beer along with the thyme sprigs, brown sugar, molasses, horseradish, salt, and pepper to the skillet. Stir to combine. Cover with lid and let cook down over medium/low heat for 20 minutes. Use your spoon to break apart the onion strands.

Add the remaining 2 ounces of beer to a small bowl along with the corn starch. mix until combined. Add the mixture to the simmering skillet. Stir to combine. Cover with lid and cook for another 20 minutes. Continue to break apart the long onion pieces.

Remove lid. Remove thyme sprigs. Increase heat to medium/high. Cook for another 10 minutes until very thick and dark in color. Once the mixture thickens up and dragging a spoon across the pan leaves a space in the jam, then the jam is done.

If you don't want big pieces of onion in the jam, add half the jam to a wide mouth mason jar or deep bowl. Use an immersion blender to chop the onion jam into more bite sized pieces. Then add that back to the skillet and stir to combine.

Once the jam is cooled to room temperature, serve with warm soft pretzels and spicy brown mustard.

Onion jam lasts up to a week in a sealed container in the fridge.