Weigh the onions if desired. You're looking for about 2 lb onions.

Get the slow cooker started on HIGH with the 1/4 cup (4 tbsp) butter inside it in small pats, so the butter can melt. In our slow cooker, it takes about 15 minutes for this to happen, so be aware you have that long to finish your prep.

Thinly slice the onions. Tie the thyme together with kitchen twine or a thin strip of cheesecloth.

When the butter is melted, add in the onions, sugar, salt, and pepper and stir well, until everything is nicely coated in the butter. Then add the thyme bundle and the bay leaf.

Cook on HIGH for 6-7 hours, stirring once halfway through (or every two hours, to prevent objectionably burnt stuck on bits for Andrew). You want the onions to be lightly carmelized when you're through.

Remove and discard the thyme and bay leaf. Stir in the broth and vinegar. Cover and cook on HIGH until thoroughly heated, about 30 minutes.

In that time, you will want to prepare cheese toasts in the broiler using the gruyere and slice the apples. Once the food is hot, it's ready to go and shouldn't need further seasoning.