Melt the butter in a large nonstick skillet over medium-high heat. Add the scallions, potatoes, 1/2 teaspoon salt, and pepper to taste. Cook, stirring occasionally, until the potatoes are golden brown and just tender, about 10 minutes; reduce the heat slightly if necessary to prevent the potatoes from burning. Add the beef and cook, stirring, until hot, about 5 minutes. Season with salt and pepper, and stir in the parsley.
Combine the chuck, bell peppers, onions, garlic, tomatoes, Italian seasoning, fennel seeds, 2 1/2 teaspoons salt, and some pepper in a slow cooker. Cook on the low setting for 8 hours. Use a ladle to remove some of the excess fat from the surface, then coarsely shred the beef with two forks. Season with salt and pepper.