Season the chuck roast generously with salt and pepper.
Heat olive oil in a large skillet over medium-high heat. Sear the roast on all sides until browned. Remove roast and set aside.
Add the sliced onions to the skillet and cook, stirring occasionally, until softened and lightly caramelized, about 10-15 minutes. Add minced garlic and cook for 1 minute more until fragrant.
Place the seared chuck roast on top of the onions in the slow cooker.
Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the roast is fork-tender.
Remove the roast from the slow cooker and shred or slice it.
Skim any excess fat from the cooking liquid. Serve the roast with the onion gravy over mashed potatoes or rice.
Notes
For deeper flavor, you can caramelize the onions further by cooking them over lower heat for a longer period.
If you prefer a thicker gravy, you can mix 2 tablespoons of cornstarch with 2 tablespoons of cold water and stir it into the cooking liquid during the last 30 minutes of cooking.
This recipe is a great example of a set-and-forget dinner, ideal for busy weeknights.