Assemble all ingredients except carrots, potatoes and horseradish. Be sure to use a good wine that you would drink.
Mince garlic and chop onion into quarters. Choose a pan that is large enough for roast to lie flat and that has a tight fitting lid.
Pat meat dry and lightly sprinkle with kosher salt and pepper. Rub flour into meat. (If using regular table salt, use even less.).
Sear in oil over medium-high heat until browned on all edges.
Add remaining ingredients except the carrots, potatoes and horseradish. If not enough liquid to cover roast, add water (or more wine).
Cover and bring to a boil, then reduce heat to low (more than simmer) and cook for 8 hours, or until the whole roast is falling apart.
The secret to tender pot roast is to cook it long enough, so if it's not tender yet, that means it's not done yet.
If you don't have enough liquid to cover the roast, you'll have to flip the roast over every hour or so.
Add carrots and potatoes the last hour. Peel them and chop to desired size.
Serve with horseradish sauce and au jus (broth from pan).