• Preheat oven to 300°F and set the racks apart so that a covered Dutch oven can fit.
• Dry off the beef cubes paper towels and season with salt. Toss the cubes into the flour then shake off the excess. Place the cubes on a parchment paper lined baking sheet. Save the remaining flour for later.
• Heat a Dutch oven to medium-high heat with the oil. Once shimmering add the beef cubes and brown on all sides. Work in batches to not crowd the pan. Place the cooked beef cubes on a plate and set aside.
• Add the onion to the pot along with a pinch of salt. If the pot is dark, add a splash of water and use a wooden spoon to scrape the pot and dislodge the brown bits. Cook the onion until soft then add the garlic and cook until golden and fragrant. Add the tomato paste and cook for 3-5 minutes. Add a splash of water if the paste starts to burn. Add a 1/4 cup of the remaining flour and cook until no white spots appear (1-2 minutes).
• Add the beef stock, balsamic vinegar, Worcestershire sauce, brown sugar, and hot red pepper flakes and bring to a boil for 2 minutes. With a wooden spoon scrape the bottom of the pot to remove all the brown bits.
• Return the beef cubes to the pot and cover. Place the pot into the oven and cook until tender (about 2 hours).
• Once the beef is tender, test for seasoning and adjust salt and pepper to taste. If more sugar is needed, mix it in now. If skipping the mushrooms, serve now, otherwise cover the pot and set aside.

For the mushrooms and pearl onions (optional)

• Heat a large pan to a touch higher than medium heat. Add the oil along with the pearl onions. Cook the onions until tender (about 4-5 minutes) then remove with a slotted spoon to a plate.
• Add the mushrooms to the same pan and cook until they release their water and start to brown (about 5-8 minutes). Once they begin to brown add the butter and cook until they are glistening (about 2-3 minutes). Return the pearl onions back to the pan and toss/mix to coat. Turn off the heat and eason with salt and pepper and add the green onion.
• You can mix all the mushrooms and pearl onions into the pot before serving or divide them onto plates so that each person gets a few pearl onions and mushrooms. Serve with mashed potatoes or crusty bread. See notes below about serving the next day which is preferred.