Line the bottom of your crock-pot with sliced onion. Salt and pepper all side of the roast and place on top of onions.
Whisk brown sugar, vinegar, worcestershire sauce, tomato paste, beef broth, garlic, and thyme together; pour over top of roast.
Cook on low for 10 hours.
In a large saucepan, melt butter. Whisk in flour and cook, whisking continuously, for five minutes.
Pour 2 cups of juice from roast into saucepan, bring to boil, and stir until thickened.
Toast two pieces of bread per serving, top with meat, and drizzle gravy over top to serve.