Season the beef pieces with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat. Add the beef and brown on all sides, about 5 minutes. Transfer the beef to the slow cooker.
Add diced onion and minced garlic to the skillet and sauté for 2-3 minutes until softened. Transfer to the slow cooker.
Pour beef broth and Worcestershire sauce into the slow cooker. Add dried thyme and dried rosemary. Stir to combine.
Cover and cook on low for 8 hours or until the beef is tender.
About 30 minutes before serving, prepare the mashed potatoes: place the peeled and cut potatoes in a large pot and cover with cold water. Add a pinch of salt and bring to a boil over high heat.
Reduce heat to medium and simmer potatoes until tender when pierced with a fork, about 15-20 minutes.
Drain the potatoes and return them to the pot. Add whole milk, unsalted butter, salt, and black pepper. Mash until smooth and creamy.
In a small bowl, mix cornstarch and cold water to create a slurry. Stir the slurry into the slow cooker beef mixture. Cover and cook on high for an additional 15 minutes until the sauce thickens.
Serve the slow cooker beef over a bed of mashed potatoes.