Prepare the beef: Pat the meat dry with paper towels and season generously with salt and pepper (about one teaspoon each).
Brown the beef: Heat the olive oil in a large skillet set over high heat. Working in batches, brown the beef on all sides, about 3-4 minutes per batch. Transfer the browned beef to the slow cooker and repeat until all of the meat has been browned.
Lightly caramelize the onions. Return the skillet to medium-high heat and add the remaining tablespoon of olive oil. Add the diced onion and cook until golden and softened, about 8-10 minutes.
Add the garlic. Stir in the minced garlic and continue to cook for another 2-3 minutes, stirring continuously.
Transfer to the slow cooker.
Prepare the bouquet garni (bundle of herbs). Tie the fresh thyme and rosemary together using cooking twine and transfer them to the slow cooker.
Deglaze the pan. Return the skillet to medium-high heat. Pour a small amount of red wine into the skillet and scrape up any browned bits.
Warm the wine sauce. Then, add the rest of the red wine, marsala wine, beef broth, and tomato paste. Gently whisk to combine and heat until warmed through.
Transfer to the slow cooker. Add the chopped carrots, mushrooms, celery, canned tomatoes, and sun-dried tomatoes to the slow cooker and carefully pour the red wine mixture on top.
Cook. Cover and cook on low for 6-8 hours or high for 4-5 hours.
Add the pearl onions about 30 to 60 minutes before serving.
Thicken with cornstarch (optional) about 15 minutes before serving. Mix the cornstarch and cold water in a small bowl, then add it to the stew.
Remove the bundle of herbs and season with additional salt and black pepper to taste. Serve hot over buttered noodles or mashed potatoes.