In large cast-iron or heavy skillet, cook bacon over medium-high heat, stirring occasionally, until crisp, about 4 minutes. Using slotted spoon, transfer to paper towel–lined plate. Discard fat.
In same pan, melt butter over medium high heat; cook onions, stirring occasionally, until softened, about 5 minutes. Add ale and sugar; cook, stirring occasionally, until no liquid remains and onions are golden brown, 30 to 35 minutes. Stir in bacon.
Pour into slow cooker. Combine garlic, salt and pepper; rub all over beef. Place on onions; pour broth over top. Cover and cook on low until beef is tender, 7 to 8 hours.
Remove beef to cutting board; tent with foil and let stand for 10 minutes before slicing.
Skim fat from cooking liquid. Whisk cornstarch with 2 tbsp. water; whisk into slow cooker. Cover and cook on high until thickened, about 10 minutes. Serve with roast.