2 tablespoons unsalted butter, at room temperature
1/2 pound white button mushrooms, quartered
Buttered egg noodles and prepared horseradish, for serving
Season the roast generously with salt and pepper and dust with flour, tapping off the excess. In a large skillet, heat 2 tablespoons of the olive oil. Add the roast and cook over moderate heat, turning occasionally, until deeply browned all over, about 20 minutes.
Transfer the roast to a slow cooker. Add the carrots, onion, garlic, beef broth and herb bundle and season with salt and pepper. Cook on high for 4 hours or on low for 8 hours, until the meat is tender.
Transfer the meat to a cutting board, cover with foil and keep warm. Pour the contents of the cooker into a colander set over a heatproof bowl; discard the herb bundle. Transfer the carrots to a bowl and keep warm.
Skim the fat off of the cooking liquid. In a small bowl, blend the butter with 2 tablespoons of flour to make a paste. Return the cooking liquid to the slow cooker and whisk in the butter paste. Cook on high until the gravy is thickened, about 10 minutes.
Meanwhile, heat the remaining 2 tablespoons of the oil in a large skillet. Add the mushrooms, season with salt and pepper and cook over high heat until browned, about 8 minutes.
Untie the roast. Slice the meat and arrange on a large, deep platter. Scatter the carrots and mushrooms all around and pour the gravy on top. Serve with egg noodles and horseradish.
The pot roast can be refrigerated for up to 4 days.