Cut off any excess fat from your roast. Combine the onion flakes, onion powder, garlic powder, dried rosemary, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a small bowl or zip-lock baggie.
Lay the roast on a piece of foil and rub on all sides with the spice mixture. Wrap up tightly in 3 layers of foil and refrigerate overnight.
Place roast (still in foil) in a slow cooker, place potatoes around the roast (cut if needed to fit in the crockpot), and cook on low for 8-10 hours. When done, carefully remove the foil and use a fork to shred if desired.
Before serving, while roast is still cooking, cut broccoli into bite-sized florets, slice mushrooms, and dice onion.
Add oil to a large skillet and heat over medium. Add vegetables and remaining salt and pepper; sauté for 5-6 minutes, stirring frequently to avoid burning.
Add vinegar and scrape any pieces off bottom of the pan. Turn heat down to low and cook, uncovered, for 5-10 minutes longer until mushrooms and broccoli are crisp-tender and onions are translucent.
Serve beef with potatoes, sautéd vegetables, and mango on the side.