Prep the Beef: In a large bowl, toss the beef cubes with the flour, salt, and pepper until they are well-coated. Shake off any excess flour.

Brown the Beef: Heat the olive oil in a skillet over medium-high heat. Add the beef in batches, making sure not to overcrowd the pan, and brown on all sides. Transfer the browned beef to the slow cooker.

Sauté Vegetables: In the same skillet, add a bit more oil if needed, and sauté the onion and garlic until soft and fragrant, about 2-3 minutes. Transfer them to the slow cooker.

Deglaze: If using red wine, pour it into the skillet and scrape up any browned bits from the bottom of the pan. Allow it to simmer and reduce slightly before adding it to the slow cooker. If not using wine, skip this step.

Combine Ingredients: To the slow cooker, add the carrots, potatoes, celery, frozen peas, diced tomatoes (with their juice), beef broth, tomato paste, thyme, rosemary, and bay leaf. Stir to combine.

Cook: Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the beef is tender and the vegetables are cooked through.

Final Touches: Remove the bay leaf and adjust seasoning with salt and pepper to taste. If the stew is too liquid for your liking, you can thicken it by mixing a tablespoon of cornstarch with 2 tablespoons of water and stirring it into the stew. Let it cook for an additional 15-20 minutes on high, or until thickened to your liking.

Serve: Ladle the stew into bowls and garnish with fresh parsley. Serve hot with crusty bread or over a bed of cooked rice or mashed potatoes.