In a large skillet, heat the olive oil over medium-high heat. Add the beef stew meat in batches (do not overcrowd the pan) and brown on all sides. Remove the browned beef and place it in the slow cooker. If you prefer to skip this step, you can add the raw beef directly to the slow cooker.

In the same skillet (if you browned the beef), add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Transfer the onion and garlic mixture to the slow cooker with the beef.

Add the condensed cream of mushroom soup, condensed cream of potato soup, beef broth, dried thyme, dried rosemary, salt, and black pepper to the slow cooker. Stir well to combine all the ingredients with the beef and aromatics. Add the cubed potatoes and frozen mixed vegetables to the slow cooker and gently stir to ensure they are well coated in the soup mixture.

Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, or until the beef is fork-tender and the potatoes are soft. The cooking time may vary slightly depending on your slow cooker. Stir occasionally during the last hour of cooking to prevent sticking and ensure even cooking.

Once cooked, give the casserole a final stir. Taste and adjust seasonings if necessary. Serve hot, garnished with fresh chopped parsley if desired. This casserole is a complete meal on its own, but it also pairs wonderfully with a side of crusty bread or a simple green salad.