Season the beef cubes with salt and pepper.
Dredge them in flour until evenly coated.
In a large pan, heat the olive oil over medium-high heat.
Fry the beef until brown on all sides, then remove and set aside.
Add pancetta to the same pan and cook until crispy.
Add chopped onion and minced garlic, stirring until onions are soft.
Return the beef to the pan.
Pour in the red wine and beef stock, then stir in tomato paste, bay leaf, and thyme.
Simmer for 1-1½ hours until the beef is tender.
Fry mushrooms and pearl onions separately, then add them to the stew for the final 30 minutes of cooking.
Remove bay leaf and thyme sprigs before serving.